A senior White House official says Moscow was notified of President Joe Biden's visit to Ukraine's capital, Kyiv, a few hours before his departure.

In a telephone news conference on Monday, White House National Security Advisor Jake Sullivan acknowledged that Biden's trip to the city was "risky."

US officials said, since there is no US military presence in Ukraine, Biden's trip differed from previous presidential visits to Iraq and Afghanistan.

Sullivan described the visit as historic and "unprecedented in modern times." He said there is "risk in an endeavor like this, and President Biden felt that it was important to make this trip."

He also said the visit had been prepared by a handful of administration officials over the course of several months. Biden was accompanied by a small group of aides and medical staff.

Sullivan added he would not disclose details on how Biden traveled to Kyiv, citing security concerns.