The White House earlier announced US President Joe Biden was scheduled to leave for Poland on Monday, ahead of the first anniversary of Russia's invasion of Ukraine on Friday. But the departure was moved forward a day and Biden entered Ukraine without notice.
The announcement said at the time Biden was expected to reaffirm unity among his country's allies and Washington's commitment to helping Ukraine.
The White House said Biden would deliver remarks on Russia's invasion, in which he would stress that the United States will continue providing support to Ukraine for as long as it takes to maintain international order.
During his trip through Wednesday, Biden was to hold talks with Polish President Andrzej Duda and the leaders of NATO's Eastern European members to reaffirm unity among US allies.
His trip comes as some members of the US Republican Party are becoming wary of providing military aid to Ukraine.
An opinion poll by the Associated Press and others in January showed that 48 percent were in favor of military assistance. That is down 12 points from May of last year.
White House National Security Council spokesperson John Kirby said last week that Biden had no plans to visit Ukraine.