US President Joe Biden is holding talks with two Ukrainian ministers in Poland - his first face-to-face meeting with such high-level officials since Russian forces invaded the country a month ago.

The White House said he would "drop by" a meeting between US Secretary of State Antony Blinken and US Defense Secretary Lloyd Austin with their Ukrainian counterparts, Dmytro Kuleba and Oleksii Reznikov.

US President Joe Biden says he thinks Russian leader Vladimir Putin has been surprised to find he hasn't been able divide Nato allies in their reaction to his invasion of Ukraine.

Biden has been speaking during a trip to the Polish capital Warsaw, which has seen him meet Ukrainian government ministers, as well as Poland's President Andrzej Duda.

"I'm confident that Vladimir Putin was counting on being able to divide Nato, being able to separate the eastern flank from the west, being able to separate nations based on past histories," Biden said.

"But he hasn't been able to do it - we've all stayed together.

"I just think it's so important that we, Poland and the United States, keep in lock step how we're proceeding."

He also says history has shown the importance of the US not remaining on the sidelines during conflict in Europe.

"We have learned from sad experience in two world wars, when we've stayed out and not been involved in stability in Europe, it always comes back to haunt us in the United States.

"So I've been saying for a long time, as senator who visited here and a member of the Foreign Relations Committee, as vice president and now as president, that stability in Europe is critically important to the United States in terms of our interest - not only in Europe but around the world."