The United States gave a heads-up to Moscow "hours" ahead of President Joe Biden's "risky" surprise trip to Kyiv on Monday to avoid any chance of conflict, a senior White House aide said.

"We did notify the Russians that President Biden will be travelling to Kyiv. We did so some hours before his departure for deconfliction purposes," National Security Advisor Jake Sullivan, who accompanied the president on the trip, told reporters. 

"Because of the sensitive nature of those communications, I won't get into how they responded or what the precise nature of our message was," he continued.

Biden arrived in Kyiv on Monday, making an extraordinary first visit to the country since Russian troops invaded on February 24, 2022.

The visit was organized in conditions of strict secrecy.

Biden left Andrews Air Force Base in the early hours of Sunday. However, White House staff said they cannot yet divulge how Biden got to the embattled Ukrainian capital.

"We are holding back on some of those mode of transportation details and other specific logistical details until the trip is fully complete," communications director Kate Bedingfield said.

Sullivan said the trip "required a security operational and logistical effort from professionals across the US government to take what was an inherently risky undertaking and make it a manageable level of risk."

"Of course, there was still risk and is still risk in an endeavour like this and President Biden felt that it was important to make this trip."

Only two journalists were allowed to accompany the president.

After handing over their devices, journalists were made aware of his presence on Air Force One just 15 minutes before the plane took off.

It was the first visit by a US president to Ukraine since 2008.