U.S. Defense Secretary Lloyd Austin made an unannounced visit to Kyiv on Monday to reassure Ukraine that Washington will continue to support its fight against invading Russian forces.

The United States has provided tens of billions of dollars in security aid for Ukraine and repeatedly pledged to back Kyiv for "as long as it takes," but opposition from hard-line Republican lawmakers has raised doubts about the future of American assistance.

Austin "traveled to Ukraine today to meet with Ukrainian leaders and reinforce the staunch support of the United States for Ukraine's fight for freedom," the Pentagon said in a statement on the trip, which was not announced ahead of time due to security concerns.