The United States has conjected that Russia has completed its pullout from areas around Kyiv and is regrouping for an offensive in Ukraine's east, a senior U.S. defense official said.

"We are assessing that all the Russians have left," the official said, speaking on condition of anonymity, adding that the U.S. assessment was completed in the past 24 hours. Despite the news of Russian troops withdrawing from around Kyiv, the official warned about the remaining threat.

The official said that while the threat of a ground invasion of Kyiv "is clearly gone for the moment," it is not clear what Russia's longer-range goals are.

Ukraine was trying to evacuate as many trapped civilians as possible on Thursday as Russian forces pounded cities and towns in the east and south of the country.

Deputy Prime Minister Iryna Vereshchuk announced an agreement had been made with Russia to open 10 safe corridors, mostly in southern and eastern Ukraine, but said residents trying to leave the besieged city of Mariupol would have to use their own vehicles. Vereshchuk urged civilians to evacuate to safer regions before it was too late.

"Later, people will come under fire, and we won’t be able to do anything to help them," Vereshchuk said.

Multiple attempts to agree safe passage for buses to take supplies to Mariupol and bring out civilians have failed since Russia's military actions began in Ukraine on Feb. 24, with each side blaming the other.

Ukrainian officials say Russia has been regrouping for a new offensive in the east and south, and want civilians to leave those areas while they still can.

"Evacuate! The chances of saving yourself and your family from Russian death are dwindling every day," said Serhiy Gaidai, the governor of the Luhansk region, adding that Russian troops had not made any significant breakthroughs.

Local officials reported heavy Russian shelling and rocket fire in the southern region of Kherson in the last few days.

Ukrainian presidential adviser Oleksiy Arestovycvh said Russian air attacks were now focused mainly on areas of eastern Ukraine, and that Mariupol was holding out. He said he believed Russian efforts to surround Ukrainian troops in the east would be in vain, adding: "The situation is under control."

'Pressure on civilian population'

Ukraine's military says Russia wants to entrench a land passage between two separatist, self-proclaimed people's republics in Donbass in eastern Ukraine and the southern region of Crimea that Russia seized and annexed in 2014.

Oleh Synyehubov, the governor of the Kharkiv region in eastern Ukraine, said civilian areas had been shelled 48 times in the past 24 hours to "put pressure on the civilian population of Kharkiv and destroy civilian infrastructure."

He said three civilians had been killed and many homes destroyed in shelling of Balakliya, a town close to the occupied city of Izyum, from where Ukraine says it can no longer evacuate civilians.

Russia denies targeting civilians in a "special military operation" which it says is aimed at demilitarizing and "denazifying" Ukraine. The Kremlin's position is rejected by Ukraine and the West as a pretext for an unprovoked invasion.

Mariupol has been under siege for most of the war. At least 160,000 civilians are trapped without power and with little food or running water, according to Mayor Vadym Boychenko, who puts the civilian death toll in the city at about 5,000.

Boychenko, who has left Mariupol, estimated that at least 40,000 residents had been forcibly deported to Russia from areas of the city occupied by Russian forces. Russia has spoken of "refugees" arriving from the strategic port city.

British defense officials said 160,000 people remained trapped in the city, which had a prewar population of 430,000. A humanitarian relief convoy accompanied by the Red Cross has tried to get into the city for days, without success. Capturing Mariupol would allow Russia to secure a continuous land corridor to the Crimean Peninsula, which Moscow seized from Ukraine in 2014.

In Brussels, Ukrainian Foreign Minister Dmytro Kuleba urged NATO to provide more weapons for his war-torn country to help prevent further atrocities like those reported in Kyiv's northern suburbs. Ukrainian authorities are working to identify hundreds of bodies they say were found in Bucha and other towns after Russian troops withdrew and to document what they say were war crimes.

"My agenda is very simple ... it’s weapons, weapons and weapons," Kuleba said as he arrived at NATO headquarters for talks with the military organization’s foreign ministers about Ukraine's fight to defend itself. "The more weapons we get and the sooner they arrive in Ukraine, the more human lives will be saved," he said.

Some NATO nations worry they may be Russia’s next target, but the alliance is striving to avoid actions that might pull any of its 30 members directly into the war. Still, NATO Secretary-General Jens Stoltenberg urged member nations to send Ukraine more weapons, and not just defensive arms.

"Ukraine is fighting a defensive war, so this distinction between offensive and defensive weapons doesn’t actually have any real meaning," he said.

Western countries have provided Ukraine with portable anti-tank and anti-aircraft weapons, but they have been reluctant to supply aircraft or tanks plus any equipment that Ukrainian troops would have to be trained to use.

Asked what more his country was seeking, Kuleba listed planes, land-based missiles, armored vehicles and air defense systems.

A U.S. defense official speaking on condition of anonymity said Russia had pulled all of its estimated 24,000 or more troops from the Kyiv and Chernihiv areas in the north, sending them into Belarus or Russia to resupply, reorganize and likely prepare to return to fight in the east.

Growing numbers of Putin’s troops, along with mercenaries, have been reported moving into the Donbass, where Russia-backed separatists have fought Ukrainian forces for eight years and control two areas.

Ahead of Feb. 24, Moscow recognized the Luhansk and Donetsk areas as independent states. Military analysts have said Putin also could be seeking to expand into government-controlled parts of the Donbass.

Donetsk Governor Pavlo Kyrylenko said at least five civilians were killed and another eight were wounded by Russian shelling on Wednesday.

Another Western official, also speaking on condition of anonymity to discuss intelligence estimates, said it may take Russia’s battle-damaged forces as much as a month to regroup for a major push on eastern Ukraine.

Oleksandr Shputun, spokesperson for the General Staff of the Ukrainian Armed Forces, reported Thursday that near Donbass, Ukraine's second-largest city, Kharkiv, remained blockaded. He said Russian forces also were carrying out "brutal measures" in the southern Kherson region, which they hold.

In his nightly address to the nation late Wednesday, Ukrainian President Volodymyr Zelenskyy said Ukraine, too, was preparing for battle.

"We will fight and we will not retreat," he said. "We will seek all possible options to defend ourselves until Russia begins to seriously seek peace. This is our land. This is our future. And we won’t give them up."

In areas north of the capital, Ukrainian officials gathered evidence of Russian atrocities amid signs Moscow’s troops killed people indiscriminately before retreating.

Ukrainian authorities said the bodies of at least 410 civilians were found in towns around Kyiv, victims of what Zelenskyy has portrayed as a Russian campaign of murder, rape, dismemberment and torture. Some victims had apparently been shot at close range. Some were found with their hands bound.

Western officials warned that similar atrocities were likely to have taken place in other areas occupied by Russian troops. Zelenskyy accused Russian forces of trying to cover up war crimes in areas still under their control, "afraid that the global anger over what was seen in Bucha would be repeated."

"We have information that the Russian troops have changed tactics and are trying to remove the dead people, the dead Ukrainians, from the streets and cellars of territory they occupied," he said in a nighttime video address. "This is only an attempt to hide the evidence and nothing more."

Switching from speaking Ukrainian to Russian, Zelenskyy urged ordinary Russians "to somehow confront the Russian repressive machine" instead of being "equated with the Nazis for the rest of your life."

He called on Russians to demand an end to the war, "if you have even a little shame about what the Russian military is doing in Ukraine."

In reaction to the alleged atrocities outside Kyiv, the U.S. announced sanctions against Putin’s two adult daughters and said it is toughening penalties against Russian banks. Britain banned investment in Russia and pledged to end its dependence on Russian coal and oil by the end of the year.

The U.S. Senate planned to take up legislation Thursday to end normal trade relations with Russia, paving the way for higher tariffs on some imports, and to codify President Joe Biden’s executive action banning imports of Russian oil.

The European Union is also expected to take additional punitive measures, including an embargo on coal.

The Kremlin has insisted its troops have committed no war crimes and alleged the images out of Bucha were staged by the Ukrainians.

Bodies were still being collected in the city. On Wednesday, The Associated Press (AP) saw two in a house in a silent neighborhood. From time to time, the muffled boom of workers clearing the town of mines and other unexploded ordnance interrupted the silence.

Workers at a cemetery began to load more than 60 bodies into a grocery shipping truck for transport to a facility for further investigation.

Police said they found at least 20 bodies in the Makariv area west of Kyiv. In the village of Andriivka, residents said the Russians arrived in early March, taking locals’ phones and detaining and then releasing some people. Others met unknown fates. Some described sheltering for weeks in cellars normally used for storing vegetables.

"First we were scared, now we are hysterical," said Valentyna Klymenko, 64. She said she, her husband and two neighbors weathered the siege by sleeping on stacks of potatoes covered with a mattress and blankets. "We didn’t cry at first. Now we are crying."