Kyiv, Ukraine - Ukraine agreed to launch peace talks with Russia on Sunday, even as battles rage on in key cities and Russian President Vladimir Putin raised the prospect of a nuclear escalation with the West.

"I don't believe in a result of this meeting, but let's try," Ukrainian President Volodymyr Zelensky said in a statement.

Delegations from each country were set to meet on Monday morning without preconditions at the Ukrainian-Belarusian border, near the Pripyat River, as the war enters its fifth day.

Zelensky had previously refused an offer by the Kremlin to meet in Belarus proper, pointing to the fact that Russia had partially staged its invasion of Ukraine out of that country.

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The announcement was seen as a positive sign but nowhere close to a breakthrough and came as Putin ordered the country's "deterrent weapons" be put on special alert, in what was seen by the West as a threatening gesture by the nuclear-armed power.

"The top leaders of the leading NATO countries are allowing aggressive statements against our country, so I order... the forces of deterrence of the Russian army on a special regime of alert," Putin said.

NATO Secretary General Jens Stoltenberg expressed alarm, saying in a BBC interview that it showed how "serious" the situation is.

A White House spokesperson said Putin was "manufacturing threats that don't exist in order to justify further aggression."

On the ground, Russian troops reportedly halted their movements in north-west Kyiv towards the capital's center.

The Ukrainian army is repelling ground and aerial attacks, the advisor to the head of the Ukrainian presidential office, Oleksiy Arestovych, said at a Sunday evening briefing, according to a statement.

He said three missiles had been fired towards Kyiv. It was not possible to independently verify the information.

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Arestovych said the Russian army has attempted to push forward into the centre of the eastern Ukrainian city of Kharkiv and that 11 Russian ships were in the sea off the coast of Odessa.

In the southern Ukrainian city of Mariupol he said there were no problems, but that Berdyansk has been taken by Russia and there were no Ukrainian army forces there.

Arestovych said more than 100,000 reservists joined the Ukrainian army over the weekend.

Fierce resistance from Ukrainian forces was slowing down the advance of Russian invading troops, according to the US Department of Defence.

For the first time, a large column of Russian military vehicles was pressing on Kyiv from the south earlier on Sunday. Previous assaults had been made from the north.

"But we know where they are heading, where they are headed, and we are prepared," Interior Ministry advisor Vadym Denysenko was quoted as saying by strana.news.

Kyiv Mayor Vitali Klitschko said his city was "not completely surrounded" but that there was fighting taking place on the outskirts and the situation remained very tense.

There was fierce fighting for control of an airfield in the town of Vasylkiv, south-west of Kyiv, according to Ukrainian sources.

According to Ukraine's armed forces, its troops repelled heavy attacks in the north-western suburbs of Hostomel and Irpin.

Kharkiv was a major focus of concern on Sunday, with residents of Ukraine's second-largest city urged to stay in their homes and shelters after invading forces had managed to break through Ukrainian defences and enter the city centre.

Videos shared on social media showed several military vehicles in the city of about 1.5 million people.

But by late afternoon the governor said Kharkiv was back under the full control of Ukrainians after forces were able to beat back the Russians.

On day four of Russia's attack, civilians once again found themselves under threat.

In Kyiv, an explosion destroyed seven cars and shattered the windows of a 16-storey apartment, the city authority reported.

It released an image showing a crater that could have been created by a grenade. DPA could not independently verify whether Russian forces were behind the attack.

In the Bucha area near Kyiv, verified video showed military vehicles shooting at homes and causing damage.

At least 210 civilians have been killed and 1,110 injured so far in Russia's war against Ukraine, parliament's commissioner for human rights told the Ukrinform news agency.

Ukraine has accused Russia of targeting civilians in its invasion of the country – a charge rejected by Moscow.

Russia on Sunday admitted to casualties in its ranks for the first time since the beginning of the war, but did not provide any numbers.

The violence has already prompted hundreds of thousands of people to flee to neighboring countries.

The international community has doubled down on its response to the invasion, which was launched in the early hours of Thursday.

A number of European allies are sending arms to Ukraine, including Germany, a major turnaround for Berlin which had staunchly refused to supply lethal weapons to Kiev for weeks in the run-up to the Russian invasion.

Sweden and Denmark said they were sending anti-tank weapons and other equipment to Ukraine. The Netherlands dispatched 200 Stinger anti-aircraft missiles, 50 anti-tank guns with 400 rockets and 100 sniper rifles with 30,000 rounds of ammunition.

The EU closed its airspace to Russian aircraft, meaning Moscow is facing a near total blockade of the continent's skies.