At peace talks with Russia on the Belarusian border, a member of the Ukrainian delegation demanded the withdrawal of all Russian troops from Ukrainian territories, including Crimea and Donbass, the deputy head of Ukraine’s presidential office Oleksii Arestovich said Monday.

Later, in comments to Ukrainian media outlets, Arestovich clarified that the demands were his personal opinion and not the official position of Ukraine.

Various sources reported that negotiations in Pripyat were going badly. Russia demanded the recognition of Crimea, self-proclaimed Donetsk People's Republic (DNR) and Luhansk People's Republic (LNR) within the boundaries of the regions and the federalization of Ukraine.

Cease-fire talks between Russian and Ukrainian officials began on the Belarusian border on Monday as Russia faced deepening economic isolation four days after invading Ukraine.

Russian forces seized two small cities in southeastern Ukraine and the area around a nuclear power plant, the Interfax news agency said.

But they ran into stiff resistance elsewhere as the biggest assault on a European state since World War Two failed to make as much ground as some had expected.

Talks began with the aim of an immediate ceasefire and the withdrawal of Russian forces, the Ukrainian president's office said. Russia has been cagier about the talks, with the Kremlin declining to comment on Moscow's aim.

It was not clear whether any progress could be achieved after Russian President Vladimir Putin put Russia's nuclear-armed units on high alert on Sunday.

The talks are being held on the border with strong Russian ally Belarus, which has become a launch pad for the invading Russian troops.

"Dear friends, the President of Belarus has asked me to welcome you & facilitate your work as much as possible. As it was agreed with the Presidents (Volodymyr) Zelenskiy and Putin, you can feel completely secure," Belarusian Foreign Minister Vladimir Makei said at the start, according to the foreign ministry's translation on Twitter.

The Western-led response to the invasion has been emphatic, with sanctions that effectively cut off Moscow's major financial institutions from Western markets. Russia's ruble currency plunged 30% against the dollar on Monday. Countries also stepped up weapons supplies to Ukraine.

Blasts were heard before dawn on Monday in the capital Kyiv and in the northeastern city of Kharkiv, Ukrainian authorities said. But Russian ground forces' attempts to capture major urban centers had been repelled, they added.

The Ukrainian General Staff said Russian forces were focusing on Chernihiv city, northeast of Kyiv, and parts of Donetsk region in the east among other areas.

Kyiv authorities warned residents emerging from a weekend curfew that they would see newly erected fortifications, tank traps and other defensive installations in the streets as the city girds for further battle.

Russia's Defense Ministry said its forces had taken over the towns of Berdyansk and Enerhodar in Ukraine's southeastern Zaporizhzhya region as well as the area around the Zaporizhzhya nuclear power plant, Interfax reported. The plant's operations continue normally, it said.

Ukraine denied that the nuclear plant had fallen into Russian hands, according to the news agency.

Dozens of people were killed in Russian rocket strikes on Kharkiv on Monday, Ukrainian interior ministry adviser Anton Herashchenko said.

The United Nations human rights chief Michelle Bachelet said at least 102 civilians in Ukraine have been killed since Thursday, with a further 304 wounded, but the real figure is feared to be "considerably higher."

More than half a million people have fled to neighboring countries, according to the United Nations refugee agency.

Fighting took place around the Ukrainian port city of Mariupol throughout the night, Pavlo Kyrylenko, head of the Donetsk regional administration, said. He did not say whether Russian forces had gained or lost ground, or provide any casualty figures.

A U.S. defense official said Russia had fired more than 350 missiles at Ukrainian targets since Thursday, some hitting civilian infrastructure.

Partners in the U.S.-led NATO (North Atlantic Treaty Organization) defense alliance were providing Ukraine with air-defense missiles and anti-tank weapons, NATO chief Jens Stoltenberg said.

The Kremlin accused the European Union of hostile behavior, saying weapons supplies to Ukraine were destabilizing and proved that Russia was right in its efforts to demilitarize its neighbor.

"Throughout Russia, the vast majority of the population has friends or relatives who live in Ukraine. Naturally, everyone's hearts are aching for what is happening to these relatives," Kremlin spokesperson Dmitry Peskov said.

The Kremlin declined to comment on whether there was a risk of confrontation between Russia and NATO. Russia has demanded that NATO never admit Ukraine.

Over the weekend, Western nations announced sanctions including blocking some Russian banks from the SWIFT international payments system.

In an emergency move, Russia's central bank raised its key interest rate to 20% from 9.5%. Authorities told export-focused companies to be ready to sell foreign currency.

It also ordered brokers to block attempts by foreigners to sell Russian securities.

Corporate giants also took action in response to the sanctions, with major British oil company the biggest foreign investor in Russia, saying it would abandon its stake in state oil company Rosneft at a cost of up to $25 billion.

Protests have been held around the world against the invasion, including in Russia, where almost 6,000 people have been detained at anti-war demonstrations since Thursday, the OVD-Info protest monitor said.

The U.N. Human Rights Council agreed on Monday to Ukraine's request to hold an urgent debate this week on the invasion after Kyiv's ambassador told the Geneva forum that some of Moscow's military actions "may amount to war crimes."

Zelenskyy on Monday asked the European Union to allow Ukraine to gain membership immediately.

"Our goal is to be with all Europeans and, most importantly, to be equal ... I am sure we deserve it," he said in a video shared on social media.

U.S. President Joe Biden will host a call with allies and partners on Monday to coordinate a united response, the White House said.

Russia calls its actions in Ukraine a "special operation" that it says is not designed to occupy territory but to destroy its southern neighbor's military capabilities and capture what it regards as dangerous nationalists.

The EU shut all Russian planes out of its airspace, as did Canada, forcing Russian airline Aeroflot to cancel all flights to European destinations until further notice.