KYIV, Ukraine — Secretary of State Mike Pompeo said on Friday that the Trump administration was committed to supporting Ukraine in its defense against aggression by Russia, which invaded and annexed part of the country and is supporting a separatist insurgency.

“Today I’m here with a clear message: The United States sees that the Ukrainian struggle for freedom, democracy and prosperity is a valiant one,” Mr. Pompeo said at a news conference after meeting with Ukraine’s leader in Kyiv. “Our commitment to support it will not waver.”

Ukraine is a “bulwark between freedom and authoritarianism in Eastern Europe,” Mr. Pompeo added.

His visit, in which he met with President Volodymyr Zelensky, was aimed at calming unease among Ukrainian officials about the relationship between Washington and Kyiv, which has been thrust into the spotlight because of the impeachment of Mr. Trump, based on charges of abuse of power and obstruction over the president’s actions on Ukraine.

Mr. Pompeo and Mr. Zelensky met before noon in the president’s office in central Kyiv, and the Ukrainian leader said they had talked about new steps to strengthen the partnership between the two nations.