"The Defence Secretary is glad that Russia has accepted the invitation to talk with his counterpart," the source said.

"Given the last defence bilateral between our two countries took place in London in 2013, Russian Defence Minister Sergei Shoigu has offered to meet in Moscow instead," added the source.

"The Secretary of State has been clear that he will explore all avenues to achieve stability and a resolution to the Ukraine crisis."

Tens of thousands of Russian troops are massed on Ukraine's border, along with an arsenal of tanks, fighting vehicles, artillery and missiles.

Russia has denied it plans to invade but the White House believes an attack could now come "at any point".

Britain's Foreign Office on Saturday updated its travel advice to Ukraine in light of the crisis.

The department now advises against all travel to two pro-Moscow separatist territories Donetsk and Lugansk, as well as Crimea, which Russia annexed in 2014.

It also advises against all but essential travel to the rest of Ukraine and said British nationals are advised to register their presence in country.

Foreign Secretary Liz Truss warned on Friday that Moscow risks becoming embroiled in a "terrible quagmire" if it invades.

In a speech in Australia, the UK's top diplomat issued a blunt and personal warning to Russian President Vladimir Putin that he is on the brink of making a major strategic blunder.

He "has not learned the lessons of history," Truss told Sydney's Lowy Institute.

"The Ukrainians will fight this, it could be a quagmire" she said.

Britain is among a handful of Western nations rushing weapons such as anti-tank missiles to Ukraine.