President Biden arrived in Poland on Monday to reinforce U.S. and NATO support for Ukraine after making an unannounced trip to Kyiv, where he recognized the one-year mark since Russia’s invasion with a show of solidarity.

The president met with Ukrainian President Volodymyr Zelenskyy to reaffirm the United States’ commitment to the Ukrainian people and announce a new round of $460 million in security assistance before departing for Poland.

“One year later, Kyiv stands. Ukraine stands. Democracy stands. America stands with you and the world stands with you. Kyiv has captured a part of my heart,” Mr. Biden said.

Mr. Biden and a small group of aides were in Kyiv for about six hours before departing on Monday afternoon local time. White House officials initially declined to detail the logistics of their transportation due to security concerns, but journalists traveling with the party later said the group traveled by train. Mr. Biden had already been scheduled to travel to Warsaw to meet with NATO allies in Eastern Europe and deliver remarks on Russia’s invasion.

In a statement announcing his trip to Ukraine, Mr. Biden said he looked forward to “traveling on to Poland to meet President Duda and the leaders of our Eastern Flank Allies, as well as deliver remarks on how the United States will continue to rally the world to support the people of Ukraine and the core values of human rights and dignity in the UN Charter that unite us worldwide.”

Poland, a NATO ally, has been an unwavering ally of Ukraine, providing both security and humanitarian assistance. The country has welcomed millions of Ukrainian refugees who have crossed the border, and provided billions of dollars in weapons and other aid to Zelenskyy’s government.

On Tuesday, Mr. Biden is scheduled to meet with Andrzej Duda, the Polish president, to discuss the United States’ cooperation with Poland and efforts to support Ukraine and NATO. Mr. Biden and Duda met in March 2022 when Mr. Biden made another trip to Poland.

Mateusz Morawiecki, the Polish prime minister, told “Face the Nation” on Sunday that the Polish government is in talks with the Biden administration about increasing U.S. troop levels in his country and establishing a “more permanent” military presence. There are currently about 11,000 U.S. troops in the country on a rotating basis.

Mr. Biden will then address his administration’s efforts to rally the world in support of Ukraine, and reiterate the U.S. “will continue to stand with the people of Ukraine for as long as it takes,” according to the White House.

On Wednesday, the president is scheduled to meet with leaders of the so-called Bucharest Nine, a group of NATO members on the alliance’s eastern front, as well as NATO Secretary General Jens Stoltenberg.

The U.S. president’s trip to Europe also comes amid new concerns, expressed publicly by Secretary of State Antony Blinken, that China is considering providing Russia with lethal aid to bolster its efforts in Ukraine.

“We have seen them provide non-lethal support to Russia for use in Ukraine,” Blinken said in an interview on “Face the Nation.” “The concern that we have now is based on information we have that they’re considering providing lethal support, and we’ve made very clear to them that that would cause a serious problem for us and in our relationship.”

The U.S. has been concerned “from day one” about the possibility that China could provide lethal support to Ukraine, Blinken said, including “everything from ammunition to the weapons themselves.”

China’s top diplomat is scheduled to visit Moscow this week. The country’s foreign ministry said in a statement over the weekend that the partnership between Russia and China “is built on the basis of non-alliance, non-confrontation and non-targeting of third countries,” and criticized the U.S. for “finger-pointing or even coercion targeting China-Russia relations.”