Russian President Vladimir Putin has warned that any Western country trying to intervene in the Ukraine war will face a "lightning-fast" retaliation as Russia stepped up its assaults on eastern and southern Ukraine, Kyiv said Thursday.

More than two months into an invasion that has flattened cities but failed to capture the capital Kyiv, Russia has mounted a push to seize two eastern provinces in a battle the West views as a decisive turning point in the war.

"The enemy is increasing the pace of the offensive operation. The Russian occupiers are exerting intense fire in almost all directions," Ukraine's military command said of the situation on the main front in the east.

It said Russia's main attack was near the towns of Slobozhanske and Donetsk, along a strategic front line highway linking Ukraine's second-largest city Kharkiv with the Russian-occupied city of Izyum. The Kharkiv regional governor said Russian forces were intensifying attacks from Izyum, but Ukrainian troops were holding their ground.

Although Russian forces were pushed out of northern Ukraine last month, they are heavily entrenched in the east and also still hold a swathe of the south that they seized in March.

Ukraine said there were strong explosions overnight in the southern city of Kherson, the only regional capital Russia has captured since the invasion. Russian troops there used tear gas and stun grenades on Wednesday to suppress pro-Ukrainian demonstrations and were shelling the entire surrounding region and attacking toward Mykolaiv and Kryvyi Rih, Ukrainian President Volodymyr Zelenskyy's southern home city, Kyiv said.

Kyiv accuses Moscow of planning to stage a fake independence referendum in the occupied south. Russian state media quoted an official from a self-styled pro-Russian "military-civilian commission" in Kherson on Thursday as saying the area would start using Russia's rouble currency from May 1.

Western countries have ramped up weapons deliveries to Ukraine in recent days as the fighting in the east has intensified. More than 40 countries met this week at a U.S. air base in Germany and pledged to send heavy arms such as artillery for what is expected to be a vast battle of opposing armies along a heavily fortified front line.

Washington now says it hopes Ukrainian forces can not only repel Russia's assault on the east but weaken its military so that it can no longer threaten neighbors. Russia says that amounts to NATO waging a "proxy war" against it.

"If someone intends to intervene in the ongoing events from the outside and create strategic threats for Russia that are unacceptable to us, they should know that our retaliatory strikes will be lightning-fast," Putin told lawmakers in St. Petersburg.

"We have all the tools for this, things no one else can boast of having now. And we will not boast, we will use them if necessary. And I want everyone to know that."

Over 1,100 bodies recovered in Kyiv

The bodies of 1,150 civilians have been recovered in Ukraine's Kyiv region since Russia's invasion and 50%-70% of them have bullet wounds from small arms, Kyiv police said Thursday.

Kyiv regional police chief Andriy Nebytov said in a video posted on Twitter that most of the bodies had been found in the town of Bucha, where hundreds of corpses have been discovered since Russian forces withdrew.

Ukraine says the civilians found dead in Bucha were killed by Russian forces during their occupation of the area.

Russia has denied targeting civilians since the Feb. 24 invasion of Ukraine. It has called allegations that Russian forces executed civilians in Bucha while they occupied the town a "monstrous forgery" aimed at denigrating the Russian army.

"To date, we have found, examined and handed over to forensic institutions 1,150 bodies of dead civilians," Nebytov said in the video, in which he stood in the rubble of buildings destroyed during heavy fighting in the Kyiv region.

"I want to emphasize that these are civilians, not military," he said.

The video was posted on the day that United Nations Secretary-General Antonio Guterres was due to hold talks in Kyiv with Zelenskyy. With the war now in its third month, Guterres on Thursday toured towns outside Kyiv, including Bucha, that have seen some of the most horrific attacks of the war.

"Civilians always pay the highest price," he said as he visited the bombed-out suburb of Irpin. "And this is something everyone should remember, everywhere in the world. Wherever there is a war the highest price is paid by civilians."

Evidence of atrocities was discovered in the towns Guterres visited on Thursday after the Russians retreated from the area in the face a fiercer than expected Ukrainian resistance, bolstered by Western arms.

Ukrainian television showed Guterres visiting the town of Borodyanka, where Zelenskyy has said the situation is "significantly more dreadful" than in nearby Bucha.

The General Staff of Ukraine's military said Russian forces were "exerting intense fire" in several places as they pushed on with the second phase of their invasion. The most intense action was around Donetsk and close to Kharkiv, which lies outside Donbass but is seen as key to Russia's apparent bid to encircle Ukrainian troops there.

The General Staff said that over the past 24 hours, the Ukrainian forces have repelled six attacks in Donbass, control of which is now Moscow's primary focus ever since its initial offensive faltered and failed to take the Ukrainian capital.

Luhansk Governor Serhiy Haidai said the Russian army shelled the residential area in his region "29 times by aircraft, multiple rocket launches, tube artillery and mortars."

An estimated 1,000 civilians are sheltering along with about 2,000 Ukrainian fighters in the steelworks, a massive Soviet-era complex with a warren of underground facilities built to withstand airstrikes.

Russia, meanwhile, said a city under its control in the south also came under fire.

'Cancerous Growth'

British Defense Secretary Ben Wallace said Putin's remarks were a sign of "almost desperation, trying to broaden this either with threats or indeed, with potential false flags or attacks."

"Having failed in nearly all his objectives," Putin was now seeking to consolidate control of occupied territory, Wallace said. "Just be a sort of cancerous growth within the country in Ukraine and make it very hard for people to move them out of those fortified positions."

Ukrainian troops are still holed up in a giant steel works in Mariupol, the ruined southeastern port where thousands of people have died under two months of Russian siege and bombardment. Putin claimed victory in the city last week, ordering the steelworks blockaded. Kyiv has pleaded for a cease-fire to let civilians and wounded soldiers escape.

"As long as we're here and holding the defense ... the city is not theirs," Captain Sviatoslav Palamar, deputy commander of Ukraine's Azov Regiment, told Reuters in a video link from an undisclosed location beneath the huge factory.

"The tactic (now) is like a medieval siege. We're encircled, they are no longer throwing lots of forces to break our defensive line. They're conducting airstrikes."

More than 5 million refugees have fled abroad since Russia launched its "special military operation" in Ukraine on Feb. 24. Moscow says its aim is to disarm its neighbor and defeat nationalists there. The West calls that a bogus pretext for a war of aggression.

U.S. President Joe Biden is expected to deliver remarks on Thursday in support of Ukrainians, the White House said.