"I am honored to be welcomed again in Kyiv. I stand in solidarity and friendship with the freedom loving people of Ukraine," Biden wrote. "Mr President, please accept my deepest respect for your courage and leadership. Slava Ukraini!"

Biden wrote the note during his 5-hour visit to the war-torn country on Monday. He used the visit to reaffirm his administration's commitment to supporting Ukraine against Russia's invasion for "as long as it takes."

US WARNED RUSSIA OF BIDEN'S VISIT TO UKRAINE ‘HOURS’ BEFORE DEPARTURE

Biden pledged an additional half a billion dollars in military assistance during the visit. The U.S. has already delivered more than $100 billion in assistance to Ukraine since Russia first invaded the country in February 2022.

UKRAINE WARNS OF MASSIVE RUSSIAN MISSILE ATTACK ON ANNIVERSARY OF PUTIN’S INVASION: ‘WE HAVE TO BE READY’

A number of U.S. lawmakers have said supporting Ukraine should be a top priority, but some have criticized decisions to continue giving financial aid without proper oversight. House Speaker Kevin McCarthy has said Ukraine should not receive a "blank check," but the Biden administration has vowed to support Zelenskyy's regime for "as long as it takes."

National security officials provided more details about Biden's surprise trip, calling the trip unprecedented given the lack of U.S. military infrastructure in the region. Reporters pressed national security adviser Jake Sullivan on whether the U.S. had warned Moscow that Biden was headed to Kyiv, a city scarred with Russian missile strikes.

"We did notify the Russians that President Biden would be traveling to Kyiv," Sullivan confirmed. "We did so some hours before his departure for de-confliction purposes."

Sullivan declined to elaborate on how the Russians were notified and whether they responded, however.