Three European leaders are expected to travel to the Ukrainian capital of Kyiv on Tuesday before a 35-hour curfew for civilians takes effect as Russian forces advance and the city is under sustained shelling.  

The prime ministers of Poland, Slovenia and the Czech Republic – all NATO member countries – are to meet with Ukrainian President Volodymyr Zelenskyy and Ukraine Prime Minister Denys Shmyhal. 

Polish Prime Minister Mateusz Morawiecki tweeted on Tuesday that he and his deputy prime minister, Jarosław Kaczyński, as well as Czech Republic Prime Minister Petr Fiala and Slovenian Prime Minister Janez Janša would be traveling to Kyiv as representatives of the European Council. 

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"Europe must guarantee Ukraine's independence and ensure that it is ready to help in Ukraine's reconstruction," he wrote. 

The Polish government also announced the trip in a statement in English on their website. 

"The visit is organized in consultation with the President of the European Council Charles Michel and the President of the European Commission Ursula von der Leyen," the government said. 

"The purpose of the visit is to confirm the unequivocal support of the entire European Union for the sovereignty and independence of Ukraine," the statement continued. "The aim of this visit is also to present a broad package of support for the Ukraine and Ukrainians. International community has been informed about the visit by international organizations, i.a. the UN." 

Meanwhile, Zelenskyy, appearing virtually, addressed leaders of the UK Joint Expeditionary Force (JEF). 

"We can still stop the killing of people and that is something we can do together — stop the destruction of democracy and do it now on our land or else they will also come to you," he said Tuesday during the meeting hosted by U.K. Prime Minister Boris Johnson. 

Leaders of Denmark, Finland, Estonia, Iceland, Latvia, Lithuania, the Netherlands, Sweden and Norway attended in-person at the Lancaster House in London.   

Zelenskyy is seeking to extend martial law until April 24 and to require men ages 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."

This comes before a 35-hour curfew is to go into effect in Kyiv on Tuesday evening. 

When Russia launched the war three weeks ago, fear of an imminent invasion gripped the Ukrainian capital, and residents slept in subway stations or crammed onto trains to flee the city. But as the Russian offensive became bogged down, Kyiv saw a relative lull. U.S. officials say Russian forces were about 9 miles from the center of the city as of Monday, according to The Associated Press. 

But large explosions thundered across Kyiv before dawn Tuesday from what Ukrainian authorities said were artillery strikes. Zelenskyy said barrages hit four multi-story buildings in the city and caused dozens of deaths. 

The early morning shelling 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-story apartment building and smoke choked 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 neighboring apartment blocks.

In a message posted to Telegram, Mayor Vitaliy Klitschko said the curfew takes effect at 8 p.m. Tuesday and ends at 7 am. Thursday. Announcing military command’s decision, he warned that movement throughout the city without special passes was prohibited unless people are headed to bomb shelters.

"Today is a difficult and dangerous moment," he said. 

He said civilians should prepare not to leave home for two days, unless sirens warning of air raids go off. Then, in that case, residents should be prepared not to leave shelters during that time-period. 

"The capital - the heart of Ukraine - will be defended!" Klitschko added. "And Kyiv, which today is a symbol and outpost of European freedom and security, we will not give up! We will not be brought to our knees! Today the efforts of every Kyivan are important." 

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Klitschko appealed to the men of Kyiv to "come back" and defend their city and said volunteers were needed to build fortifications and prepare and deliver food for defenders and vulnerable citizens. 

"Men of Kiev! Come back! We need to protect our city and our future!" he said. "Instead of sitting somewhere and sympathizing! Ukrainians have shown the world their courage and willingness to fight for their land. Only together we will survive! Glory to Ukraine!" 

The U.N. has described the flood of people crossing into Poland and other neighboring countries as Europe’s biggest refugee crisis since World War II.

The International Organization for Migration revealed on Tuesday that more than 3 million people have fled Ukraine since Russia invaded, according to The Associated Press. 

Russian and Ukrainian negotiators also planned to hold a second day of talks as Russia’s invasion of Ukraine reached its 20th day. The Ukrainian government said new aid and evacuation efforts would take place Tuesday along nine corridors around the country, including the Kyiv region. But past evacuation attempts have repeatedly failed amid continued fighting.

The International Committee of the Red Cross said an evacuation involving 30 buses was planned from Sumy in northeast Ukraine. The organization said it still had not gotten aid to 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 Associated Press contributed to this report.