Appearing in a war zone under regular attack, White House officials describe Joe Biden's unexpected visit to Ukraine's capital Kyiv as "unprecedented in modern times".

They say previous presidential trips to wartime Iraq and Afghanistan had the back up of a heavy US military presence.

And despite widespread speculation among the press corps that Mr Biden might be planning a trip to Ukraine while he was in Poland, the visit still took everyone completely by surprise.

The sight of him appearing beside President Volodymyr Zelensky in the heart of Kyiv and under the sound of air raid sirens, makes a louder statement than anything he can say in a speech in Poland.

"It was risky and should leave no doubt in anyone's mind that Joe Biden is a leader who takes commitment seriously," said White House communications director Kate Bedingfield.

No phones

Mr Biden had been scheduled to fly out from the US to Warsaw on Monday evening, for a two-day trip.

The advance schedule had two suspiciously lengthy gaps in his itinerary, and many wondered if that might be when he would slip into Ukraine.

Reporters at the daily White House press briefings have been repeatedly asking about a visit. We were told that there was no meeting scheduled with Mr Zelensky and no stops planned outside Warsaw "right now".

The final decision to make the trip to Kyiv was only taken on Friday, even though it had been planned for months with a handful of the presidents' top aides.

On Sunday, the official White House schedule still showed the president taking off for Warsaw at 19:00 EST (00:00 GMT) on Monday evening. In fact, Air Force One took off at 04:15 EST on Sunday morning.

On board was a deliberately small team of his closest aides, a medical team and security officers.

Only two journalists were allowed to travel with the president. They were sworn to secrecy and had their mobile phones taken away from them. They were not allowed to report the visit until after Mr Biden had arrived in Kyiv.

Russia was notified of the trip a few hours before Mr Biden's departure, according to the US national security adviser Jake Sullivan.

He said the US "did so for deconfliction purposes… I won't get into how they responded or what the precise nature of our message was, but I can confirm we provided that notice".

Vivid message home

President Biden then spent 10 hours on a train to get to Kyiv. He could have visited other locations inside Ukraine that would have been easier to get to, but he wanted to make the symbolic trip to Kyiv itself.

While the president's trip is a signal to Moscow of the Biden administration's commitment to helping Ukraine, it is also a demonstration to US voters back home.

His press secretary, Karine Jean-Pierre, was asked last week about polls that show American support Ukraine softening.

She answered that whenever the president speaks, he is talking to the American people as well as people around the world. Monday's message is designed to vividly counter the minority of Republican voices who question for how long the US can continue supporting Ukraine.

Meanwhile, the surprise visit has left reporters like me, who thought they were travelling with Mr Biden later on Monday, making the trip to Poland on an aircraft that will not be called Air Force One.

The world-famous call sign is only used when the president is on board.