Poland's Donald Tusk arrived in Kyiv by train on Monday morning, Polish Radio's polskieradio24.pl website reported.
He is set to meet with Ukraine's President Volodymyr Zelensky and Prime Minister Denys Shmyhal, among others, according to officials.
The Polish prime minister first mentioned his planned visit to Ukraine in a media interview on January 12, polskieradio24.pl reported.
Tusk said that for him and Poland's President Andrzej Duda, developments in Ukraine were "issue number one due to Poland's national security."
The prime minister added that he would also discuss with Ukrainian leaders "issues to do with the interests of Polish hauliers."
Monday is day 698 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Source: IAR, polskieradio24.pl, PAP