Ben Wallace is meeting fellow NATO defence ministers in Brussels on Wednesday as efforts continue to avert a war in Ukraine with a 3am expected invasion deadline having come and gone.
The UK defence secretary will join NATO Secretary General Jens Stoltenberg and US Defence Secretary Lloyd Austin as the alliance considers how to react to the 130,000 Russian troops massing at Ukraine's borders.
The meeting comes after Vladimir Putin said on Tuesday that Russia did not want another war, and was open to further dialogue with the US and its NATO allies.
In the UK, armed forces minister James Heappey said he was cautiously optimistic about news some troops were withdrawing from the Ukrainian border but added he would "continue to be very vigilant" of Russia's actions.
The prime minister had earlier suggested there were "mixed signals" coming out of Russia about the prospect of an invasion of Ukraine, describing the situation as "not encouraging".
President Joe Biden, meanwhile, gave a televised address in the United States, in which he said an invasion was still possible and stressed the US would defend all NATO territory.
According to reports, US intelligence sources believed an invasion could commence at 3am local time - 1am in the UK - on Wednesday, however this never happened.
The ex-US ambassador to Ukraine John Herbst, who was posted to the country between 2003 and 2006, said the Russians were "saying two contradictory things officially".
Mr Herbst told the BBC : "Moscow is directly threatening an invasion. Many times Russian officials have said that unless Russian demands are met, they will take military technical measures against Ukraine. So the Russians are saying two contradictory things officially.
"The buildup of now over 150,000 Russian troops on Ukraine's borders and a flotilla of 30 or 40 Russian ships near Ukrainian territorial waters in the Black Sea, suggests intimidation at a minimum."
Heappey welcomed news Russia was withdrawing troops, saying it gave the UK "some real cautious optimism".
He told Channel 4 News: "If, as a consequence of a combination of things over the last few weeks, the threat of economic sanctions and the impact that that threat is having on the Russian economy, if the cost of military action has started to become clear, then that is a good thing that Russia may be moving in a different direction."
He added: "But until those are matched by troop movements, people like me need to continue to be very vigilant and be very clear in what we are saying in communicating what we are seeing and the danger that that presents."
Following a Cobra emergency committee meeting, Boris Johnson said the intelligence he has received about Russian military activity is "not encouraging", with the construction of field hospitals and the movement of extra forces closer to the border suggesting preparations are still being made for an invasion.
While Johnson acknowledged Russia had claimed it was withdrawing troops from the border, he added there were "more battalion tactical groups being brought closer to the border".
"So, mixed signals, I think, at the moment," he said.
Speaking after talks with German Chancellor Olaf Scholz, Putin said Russia was ready for dialogue and emphasised the need for the West to heed his main demands.
He claimed the US and NATO had rejected Moscow's demand to keep Ukraine and other ex-Soviet nations out of NATO, and to roll back alliance forces from eastern Europe.
The statement followed the Russian defence ministry's announcement of a partial pullback of troops after military drills, adding to hopes the Kremlin might not invade Ukraine imminently.
However, the Russian military did not give details of where the troops were pulling back from, or how many troops were involved.
