U.S. officials warned Russia that President Biden was visiting the Ukrainian capital of Kyiv hours before he made the trip, the White House National Security Council revealed Monday.

National security officials provided more details about Biden's surprise trip to Ukraine on Monday, calling the trip unprecedented given the lack of U.S. military infrastructure in the war-torn 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 told reporters Monday. "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.

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Biden meets Volodymyr Zelenskyy in Kyiv, Ukraine. (WHD Photo/Evan Vucci, Pool)

The U.S. warned Russia that President Biden was visiting Ukraine on Monday. (Sputnik/Mikhail Metzel/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY.)

Sullivan went on to characterize the trip as an "affirmation" of the Biden administration's commitment to supporting Ukraine.

"This was not a celebration," he clarified, adding that there will likely be dark days ahead.

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Biden himself acknowledged the challenges facing Ukraine in comments during his visit. Large areas of Ukraine remain under Russian control even after months of successful ground offensives from Kyiv's forces.

"Over the last year, the United States has built a coalition of nations from the Atlantic to the Pacific to help defend Ukraine with unprecedented military, economic, and humanitarian support – and that support will endure," Biden said in a Monday statement.

Sirens could be heard in war-torn Ukraine as Biden arrived in the country. The U.S. has already supported Ukraine with tens of billions of dollars in financial aid and military equipment.

President Biden visited the war-torn country of Ukraine on Monday. (ED JONES/AFP via Getty Images)

President Joe Biden, center, poses with Ukrainian President Volodymyr Zelenskyy, right, and Olena Zelenska, left, spouse of President Zelenskyy, at Mariinsky Palace during an unannounced visit in Kyiv, Ukraine. (WHD Photo/Evan Vucci, Pool)

Japanese Prime Minister Fumio Kishida also announced $5.5 billion in aid for Ukraine as Biden visited with Zelenskyy.

Biden's visit comes just before Russia is expected to ramp up its offensive once again in its own acknowledgment of the one-year anniversary of the conflict. Ukrainian officials warned of a massive barrage of missiles that Russia may launch on Friday, Feb. 24.

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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."