US secretary of state Antony Blinken said on Wednesday that Russia could launch a new attack on Ukraine at very short notice but Washington would pursue diplomacy as long as it could.

On a visit to Kiev to show support for Ukraine, the top US diplomat said Ukrainians should prepare for difficult days. He said Washington would keep providing defence assistance to Kiev and renewed a promise of severe sanctions against Russia in the event of a new invasion.

The Kremlin said tension around Ukraine was increasing and it was still waiting for a written US response to its sweeping demands for security guarantees from the West.

The pessimistic statements highlighted the gulf between Washington and Moscow as Blinken gears up for a meeting with Russian foreign minister Sergei Lavrov on Friday.

Blinken promised “relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace”. He said a Russian build-up of tens of thousands of troops near the Ukrainian border was taking place with “no provocation, no reason”.

“We know that there are plans in place to increase that force even more on very short notice, and that gives President (Vladimir) Putin the capacity, also on very short notice, to take further aggressive action against Ukraine,” he said.

Independent security analysts say they do not believe Moscow has so far assembled the logistics and medical units it would need to launch an immediate attack