Russia is sending “mixed signals”, the British Prime Minister, Boris Johnson, said on Tuesday after chairing a meeting of the UK government’s “Cobra” committee which is convened to deal with national and international emergencies.

He appeared specially concerned that Russia was building field hospitals near the border with Ukraine, and gave more detail on planned sanctions. But he added that there was still an “avenue for diplomacy” and if the “positive signs” were correct “let’s build on that”.

Boris was asked about a Russian statement that some of its troops and tanks were being withdrawn from the border with Ukraine and sent home in the first apparent sign of de-escalation.

The Prime Minister said: “Last night going into today, clearly there are signs of a diplomatic opening, there always has been an opportunity to talk.

“There are grounds for a conversation about Ukraine, with Ukraine, and that is good, we are seeing a Russian openness to conversations.

“On the other hand the intelligence we are seeing today is still not encouraging. We have got Russian field hospitals being constructed near the border with Ukraine in Belarus, only can be construed as a preparation for an invasion.”

He spoke after being briefed by British intelligence: “You have got more battalion tactical groups actually being brought closer to the border with Ukraine, according to the intelligence that we are seeing, so mixed signals I think at the moment and all the more reason therefore for us to remain very tough and very united and particularly on the economic sanctions.”

In London, there has been talk for some time – inspired, it has to be said, mainly by reports coming from America – that President Putin has earmarked Wednesday, February 16, as the day when he would invade Ukraine.

Asked whether an invasion could still happen in the next 24 hours, Boris said: “We think that they have a huge preparation ready to go at virtually any time, 130,000 troops or more with a huge number, more than 90 battalion tactical groups, and they are stationed around the Ukrainian border.

“Everybody can see what the potential routes in are, down to the south from Belarus, encircling the Ukrainian army in the east around the area, the enclave in Dombast, or even coming up from the south from the sea, taking Odessa, Kherson, that is the kind of thing they could do, there are a lot of options that they have.”

The PM went on: “I think what everybody needs to see is a programme of de-escalation and that means withdrawing the battalion tactical groups away from the potential theatre of conflict, not constructing field hospitals on the border between Belarus and Ukraine, a sense that things are being scaled back, scaled down, that the threat is over and that a conversation and negotiation is beginning.”

He also made it clear that the British embassy in the Ukraine capital of Kiev, under its ambassador Melinda Simmons, would stay open for as long as possible.

Boris rejected criticism his government had not done enough to stem the flow of “dirty Russian money” through the City of London. It is also the case that British nationals of Russian origin – some apparently close to Putin – have been generous donors to Tory party coffers.

Boris said he wanted to “unpeel the facade of Russian ownership of companies”.

“We’ve been out in the lead in tackling global corruption,” he said, adding he that the UK planned to strengthen measures against ill-gotten Russian money.