Russian shelling struck Kyiv on Tuesday killing at least two people, authorities said, as invading forces tightened their grip on the Ukrainian capital and the mayor announced a 35-hour curfew starting at 8 p.m. (1800 GMT).

Two large blasts echoed across the centre of the city just before dawn on Tuesday. Late on Monday, tracer bullets flashed across the night sky as Ukrainian forces apparently targeted an enemy drone.

"Today is a difficult and dangerous moment," mayor Vitali Klitschko said.

"The capital is the heart of Ukraine, and it will be defended. Kyiv, which is currently the symbol and forward operating base of Europe’s freedom and security, will not be given up by us."

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Reuters witnesses saw a high-rise apartment block in flames after being struck by artillery. Firefighters tried to douse the blaze and rescue workers helped evacuate residents trapped inside using mobile ladders. A body lay on the ground in a bag.

The prime ministers of the Czech Republic, Poland and Slovenia were travelling to Kyiv by train on Tuesday to show support for the country on behalf of the EU, the first foreign leaders to visit the Ukrainian capital since Russia invaded last month.

Polish Prime Minister Mateusz Morawiecki said on Facebook the trip took place on the 20th day of Russian President Vladimir Putin's Putin's "criminal aggression against Ukraine".

"At such breakthrough times for the world, it is our duty to be where history is forged. Because it's not about us, but about the future of our children who deserve to live in a world free from tyranny," he said.

Top aide to Morawiecki, Michal Dwoczyk, told reporters the delegation had crossed the Polish-Ukraine border by train after 8 a.m. (0700 GMT).

"The purpose of the visit is to confirm the unequivocal support of the entire European Union for the sovereignty and independence of Ukraine," Czech Prime Minister Petr Fiala said in a statement also released by the Polish government.

"The aim of this visit is also to present a broad package of support for Ukraine and Ukrainians."

Slovenian Prime Minister Janez Jansa was also taking part in the trip, which Fiala said was taken in consultation with European Council President Charles Michel and European Commission President Ursula von der Leyen.

The idea of the trip was agreed at an EU leaders' summit in Versailles in France last week, Dworczyk said.

Morawiecki was to be joined by Polish Deputy Prime Minister Jaroslaw Kaczynski, leader of the country's ruling PiS party.

The Czech Republic and Poland, former communist members of both the EU and NATO, have been among the strongest backers of Ukraine in Europe since the Russian invasion.

Russia calls its actions a "special military operation" to "denazify" Ukraine, which Kyiv and its Western allies reject as a pretext for an unjustified and illegal attack.

Kyiv has been spared the worst of the fighting since Russia invaded Ukraine on Feb. 24, but the Russian military is slowly closing in on the city and the shelling has intensified.

"What is happening right now in Kharkiv, in Mariupol and other cities - it was understandable that sooner or later it would happen in Kyiv," said local resident Igor Krupa.

Sitting on the ground outside the badly damaged building, he described how he had cocooned himself with furniture and metal weights before going to sleep.

"This actually saved me because all the windows went out and all the debris went into the apartment, and I remained unwounded. Just a couple of scratches."

In another part of the city, residents cleared debris from their homes after shelling blew out windows, ruined balconies and left wreckage strewn across the ground.

Thousands of people have been killed in the conflict and millions more displaced.

Russia, which denies targeting civilians, calls its actions a "special military operation" to "denazify" the country, a claim that Ukraine and its allies reject as a pretext for an unjustified and illegal attack.