Russia’s bombardment of Ukraine has edged closer to central Kyiv as a series of strikes hit a residential area.

The barrage continued as the leaders of three European Union countries planned a bold visit to Ukraine’s capital, with the number of people driven out of the country by war passing three million.

The White House has confirmed Joe Biden will travel to Brussels next week for face-to-face talks with European leaders over the invasion, as Nato secretary-general Jens Stoltenberg convenes a summit on the matter, to be led by the US president.

Large explosions thundered across Kyiv before dawn on Tuesday from what Ukrainian authorities said were artillery strikes, as Russia’s assault on the city appeared to become more systematic.

Ukrainian President Volodymyr Zelensky said barrages hit four multi-storey buildings in the city and caused dozens of deaths.

The strikes targeted a western district of Kyiv, disrupting a relative calm that returned after an initial advance by Russian forces was stopped in the early days of the war.

Tuesday’s shelling ignited a huge fire in a 15-storey apartment building and spurred a frantic rescue effort.

Kyiv mayor Vitali Klitschko announced a new curfew for the city, saying it would run for 35 hours from 8pm on Tuesday until 7am on Thursday (6pm GMT on Tuesday to 5am GMT on Thursday).

Meanwhile, civilians in 2,000 cars fled Mariupol along a humanitarian corridor in what was believed to the biggest evacuation yet from the desperately besieged seaport.

As Russia stepped up its assault on Kyiv, the leaders of Poland, the Czech Republic and Slovenia set out for Ukraine’s embattled capital by train to show support for the country.

“The aim of the visit is to express the European Union’s unequivocal support for Ukraine and its freedom and independence,” Czech Prime Minister Petr Fiala said in a tweet.

He was joined by his counterparts Janez Jansa of Slovenia, Mateusz Morawiecki of Poland and Jaroslaw Kaczynski, Poland’s de-facto leader.

EU officials characterised the visit as one the central European leaders had undertaken independently despite security risks. The 27-nation bloc’s other leaders were informed of the trip but did not sanction it, EU officials said.

The International Organisation for Migration said the number of people who have fled Ukraine since Russia invaded on February 24 passed three million on Tuesday.

The UN has described the flood of people crossing into Poland and other neighbouring countries as Europe’s biggest refugee crisis since the Second World War.

Russian and Ukrainian negotiators held a second day of talks as Russia’s invasion of Ukraine reached its 20th day.

The Red Cross and the United Nations refugee agency say millions of people face food and medicine shortages along with the immediate conflict threats of shelling and air attacks.

The Ukrainian government said new aid and evacuation efforts would take place on Tuesday along nine corridors around the country, including the Kyiv region. But past attempts have repeatedly failed amid continued fighting.

One of the most desperate situations is in Mariupol, an encircled port city of 430,000 where local officials estimate a lethal siege has killed more than 2,300 people and left residents desperate for food, water, heat and medicine.

The Mariupol city council reported on Tuesday that civilian evacuations were under way. The council said 2,000 civilian cars had managed to leave along a humanitarian corridor that runs for more than 260 160 miles west to the city of Zaporizhzhia.

The city council said another 2,000 cars were waiting to leave along the evacuation route. It was not immediately clear if the number of departed vehicles given on Tuesday included 160 cars that left the day before.

Meanwhile, the EU has slapped sanctions on Chelsea FC owner Roman Abramovich as part of a new package of measures targeting Russia.

The EU included the Russian oligarch in its updated list of individuals facing assets freeze and travel bans over their role in the Russian invasion of Ukraine.

The 55-year-old aluminium magnate was among seven wealthy Russians who had their assets frozen under British sanctions in response to Russia’s invasion of Ukraine.

Abramovich has also been suspended as director of the Premier League club.

The EU said Abramovich “has had privileged access to the (Russian) president, and has maintained very good relations with him. This connection with the Russian leader helped him to maintain his considerable wealth”.

Russia’s invasion has shocked the world, upended Europe’s post-Cold War security order and driven millions from their homes. Russia’s military is bigger and better equipped than Ukraine’s, but its troops have faced stiffer-than-expected resistance, bolstered by arms supplied by the West.

US officials said Russian forces are about nine miles from the centre of the city as of Monday.

Fighting has intensified on Kyiv’s outskirts in recent days, and sporadic air raid sirens ring out around the capital.

The early morning artillery strikes hit the Svyatoshynskyi district of western Kyiv, adjacent to the suburb of Irpin, which has seen some of the worst battles of the war.

Flames shot out of the 15-storey apartment building and smoke filled the air as firefighters climbed ladders to rescue people.

The assault blackened several floors of the building, ripped a hole in the ground outside and blew out windows in neighbouring apartment blocks.

Rescue workers said one person died and several were rescued, but others remain inside.

Shockwaves from an explosion also damaged the entry to a city centre subway station that has been used as a bomb shelter.

City authorities tweeted an image of the blown-out facade, saying trains would no longer stop at the station.

A 10-storey apartment building in the Podilsky district of Kyiv, north of the government quarter, was damaged by unspecified ammunition.

Russian forces also stepped up strikes overnight on Irpin and the north-west Kyiv suburbs of Hostomel and Bucha, said the head of the capital region, Oleksiy Kuleba.

“Many streets (in those areas) have been turned into a mush of steel and concrete. People have been hiding for weeks in basements, and are afraid to go out even for evacuations,” Mr Kuleba said on Ukrainian television.

In the country’s east, Russian forces launched more than 60 strikes overnight on Ukraine’s second-largest city, Kharkiv, according to regional administration chief Oleh Sinehubov.

The strikes hit the city’s historical centre, including the main marketplace.

Mr Sinehubov said fires were raging and rescuers had pulled “dozens of bodies of civilian residents”, from the ruins of destroyed apartment buildings.

Mr Zelensky is seeking to extend martial law until April 24 and to require men aged 18 to 60 to stay in the country to fight. Ukraine’s parliament is expected to vote on the measure this week.

The Ukrainian leader appealed for more weapons to counter Russia’s military. He said Ukraine’s forces are rapidly using up weapons and other hardware supplied by Western nations, and he asked northern European leaders to “help yourself by helping us”.

Meanwhile, US administration officials have alleged that Moscow has asked China for help as it attempts to make headway, and Beijing had signalled that it would be willing to provide both military support and financial backing to help stave off effects of Western sanctions.

Russia and China denied military assistance had been asked for or granted.

Talks between Russian and Ukrainian negotiators resumed on Tuesday.

The two sides had expressed some optimism about the negotiations, which Ukrainian presidential aide Mykhailo Podolyak said were discussing “ceasefire” and “withdrawal of troops from the territory of the country”.

The Ukrainian general prosecutor’s office released details of two deadly Russian attacks that took place on Monday: an artillery strike that hit a university and open-air market in the northern city of Chernihiv, killing 10, and the shooting of a 65-year-old woman on a bus that was evacuating civilians from a Kyiv suburb.

The number of people killed in a Russian rocket attack on a TV tower in western Ukraine on Monday rose to 19, authorities in the Rivne region said Tuesday.

A further nine people were injured in the strike on the TV tower in Antopol, a village about 160 100 miles from the border of Nato member Poland.