One of President Joe Biden's top aides denied Russian claims that the Kremlin guaranteed his safety ahead of a clandestine trip to Kyiv on Monday. 

National Security Adviser Jake Sullivan confirmed that Moscow had been informed of the president's planned travel to avoid any misunderstandings.

'They acknowledged receipt,' he told reporters in Warsaw, Poland, a day later.

'End of story.'

Biden's visit was cloaked in secrecy. He flew out of Washington early on Sunday morning, before landing in Poland and making the 10-hour train journey to  Ukraine's war-torn capital.

He promised more weapons for Ukraine and delivered a publicity coup for Ukrainian President Vlodomyr Zelensky, much to Moscow's fury.

Russian Security Council Deputy Chairman and former Prime Minister Dmitry Medvedev wrote on his Telegram channel: 'Biden, having previously received security guarantees, finally went to Kyiv. 

'He promised many weapons and swore allegiance to the neo-Nazi regime to the grave.

And, of course, there were mutual incantations of victory, which will come with new weapons and courageous people.'

But it was nonsense to suggest that any guarantees were made, said Sullivan, who accompanied Biden on the trip. 

He explained that Moscow had been notified to ensure they did not misunderstand any increase in security activity in the region.

'Just to let them know he would be there in this time period,' he said.

'And the means by which he was travelling, and then he would be out on this timetable, the means by which he was travelling out. 

'We conveyed that information.'

The Russians did not respond, he said 'other than to acknowledge receipt of the notification.'

The White House revealed a day earlier that it warned the Kremlin before Biden's trip. 

Sullivan said it was for 'deconfliction purposes' before Biden headed into an active war zone where the U.S.

has no military presence and only some embassy staff. 

Some MAGA-aligned Republicans back home grumbled that Biden made this journey instead of visiting East Palestine, Ohio, the site of a toxic train derailment earlier this month.