The leaders will meet with Ukrainian President Volodymyr Zelenskyy and Prime Minister Denys Shmyhal in Kyiv. A European Union aid package is also expected to be presented.

The leaders of three European countries are traveling by train to Ukraine to pledge their support.

The prime ministers of Poland, the Czech Republic and Slovenia are due to arrive in Kyiv on Tuesday to express European Union solidarity as Russia continues its assault on Ukraine's capital city.

The leaders are set to meet with Ukraine's President Volodymyr Zelensky and Prime Minister Denys Shmyhal, and are going in their capacity as representatives of the European Council.

Europe must 'guarantee Ukraine independence'

Polish Prime Minister Mateusz Morawiecki said in a tweet that "Europe must guarantee Ukraine's independence and ensure that it is ready to help in Ukraine's reconstruction."

According to Poland's presidential aide Michal Dworczyk, the leaders are expected to present an EU aid package for Ukraine.

"In Kyiv, in addition to a strong signal of support for Ukraine, a concrete support package will be presented by the prime ministers," Dworczyk said.

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

The three countries are all EU member states and also members of the NATO alliance. Over the weekend, EU leaders made it clear that there could be no shortcut to EU membership, following a two-day informal summit at the Palace of Versailles in France.

The leaders of the 27 EU member states did pledge military aid amounting to €1 billion ($1.1 billion).

At last week's two-day summit, European Commission President Ursula von der Leyen said Ukrainians were exercising "their right to choose their own destiny" by starting the process to join the EU.