Prime Minister Mateusz Morawiecki of Poland announced he was joined by Petr Fiala of the Czech Republic and Janez Jansa of Slovenia. Joining them is Jaroslaw Kaczynski, Poland's deputy prime minister for security but also the leader of the conservative ruling party, Law and Justice, a position which allows him to be the most powerful politician in Poland.
They are to meet Ukrainian President Volodymyr Zelenskyy and Prime Minister Denys Shmyhal.
Morawiecki said on Facebook that he and the other leaders were making their trip in agreement with the European Union, citing the chairman of the Council of the European Union, Charles Michel, and the head of the European Commission, Ursula von der Leyen. He said the United Nations had been informed of the visit.
``In such critical times for the world it is our duty to be where history is forged,'' Morawiecki said. ``Because it's not about us, but about the future of our children who deserve to live in a world free from tyranny.''
The visit had been planned for several days but was kept secret for security reasons, said Michal Dworczyk, the head of Morawiecki's office.
He said that a proposal of concrete help for the nation would be presented to Ukraine's leaders.
Shortly before dawn, large explosions thundered across Kyiv from what Ukrainian authorities said were artillery strikes. The shelling ignited a huge fire and a frantic rescue effort in a 15-story apartment building. At least one person was killed and others remain trapped inside.
Shockwaves from an explosion also damaged the entry to a downtown 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.
Ahead of his departure, Morawiecki on Facebook recalled how the former Polish President Lech Kaczynski had made a visit to the capital of Georgia in 2008 when that ex-Soviet country was under attack from Russia.
He quoted President Kaczynski who said at the time in Tbilisi: ``Today Georgia, tomorrow Ukraine, the day after tomorrow the Baltic states, and then maybe it's time for my country, for Poland.''
Jaroslaw Kaczynski's presence also has symbolic significance. He is the surviving twin of Lech Kaczynski, who died in a plane crash with 95 others on Russia soil in 2010 while traveling to commemorate Poles executed by the Soviet secret police during World War II. A Polish investigation determined that plane crash to be an accident.