"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 the country.

- 'Quagmire' -

Few military experts believe that Kyiv's smaller forces -- although rapidly modernising -- could repel an outright Russian invasion.

But British Foreign Secretary Liz Truss warned on Friday that Moscow still 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.

Russia has put pressure on Ukraine since an uprising nearly a decade ago toppled a government that had resisted calls to move closer to the West.

Moscow seized the Crimean peninsula in 2014 when a pro-Russian insurgency broke out in eastern Ukraine that has since claimed more than 13,000 lives.

Ukraine's calls to Western allies to bolster its defence capabilities have seen the United States, Britain and Baltic states agree to send Kyiv weapons, including anti-tank and anti-aircraft missiles.

But Ukraine's Foreign Minister Dmytro Kuleba on Saturday slammed Germany for its refusal to supply weapons to Kyiv, urging Berlin to stop "undermining unity" and "encouraging Vladimir Putin".

Kuleba said on Twitter that Germany's statements "about the impossibility of supplying defence weapons to Ukraine" did not match "the current security situation".

Ukraine's minister stressed that "today the unity of the West in relation to Russia is more important than ever".

Ukraine is "grateful" to Germany for the support it has already provided, but its "current statements are disappointing," he added.

Earlier on Saturday, German Defence Minister Christine Lambrecht said that Berlin will send a field hospital to Ukraine, while once again rejecting Kiev's calls for weapons.

Moscow insists it has no plans to invade Ukraine but has at the same time laid down a series of security demands -- including a ban on Ukraine joining NATO -- in exchange for de-escalation.