Boris Johnson has said there are “mixed signals” signals coming from Russia over whether it intends to invade Ukraine.
The prime minister said while there were “signs of a diplomatic opening” the latest intelligence he had seen was “not encouraging”.
He was speaking following a meeting of the government’s emergency Cobra emergency committee on Tuesday morning.
Russia has massed more than 130,000 troops near Ukraine. But Moscow denies it has any plan to invade.
Earlier, Russia’s defence ministry said some of its troops in the region were returning to base following the completion of military exercises.
But Johnson said the construction of Russian field hospitals close to Belarus’ border with Ukraine could only be “construed as preparation for an invasion”.
“Last night going into today, clearly there are signs of a diplomatic opening,” he said.
“On the other hand the intelligence we are seeing today is still not encouraging.”
Asked by broadcasters about the reported view of the US that Russia could launch an invasion at 1am on Wednesday morning, Johnson said: “We think that they have a huge preparation ready to go at virtually any time.”
Despite Russian claims about troop withdrawals, Johnson said intelligence suggested “you have got more battalion tactical groups being brought closer to the border”. He added: “So, mixed signals, I think, at the moment.”
Nato secretary-general Jens Stoltenberg also told reporters on Tuesday that the alliance had “not seen any de-escalation on the ground”.