The prime minister will "ramp up deterrence to avoid bloodshed" between the two former Soviet states and is determined to "accelerate diplomatic efforts" during the trip, a Downing Street spokeswoman said.

"He will reiterate the need for Russia to step back and engage diplomatically when he speaks to President Putin this week," she added.

The move comes as Ukrainian reservists, some clutching mock wooden rifles, have taken part in military exercises on the outskirts of the country's capital Kyiv.

Ukraine has launched a new territorial defence force this year, which it wants to build up into a corps of up to 130,000 people, as nervousness over the threat of some 120,000 Russian troops near its border grows.

Among the reservists are a lawyer, an IT programmer, and a freelancer in online advertising.

While they may stand little chance against the much bigger and better-equipped professional Russian army, reservists like them could be tasked with protecting civilian sites in Kyiv amid any conflict.

'Our Cuban missile crisis moment'

Tobias Ellwood, chair of the Commons defence select committee, told Sky News the tensions are "our Cuban missile crisis moment, and we must not blink".

He said: "From a Russian perspective, there's never been a better time to invade Ukraine - something Putin has been wanting to do for a long time.

"He's enjoying this international attention. Every time a leader puts their hand up and says 'I'm going to go to Moscow', of course he's going to say yes.

"He's spinning this out. I doubt whether an invasion will take place before the Beijing Olympics, China would have made sure of that."

'He absolutely will do it': Why Putin seems poised to attack Ukraine - after one of surest signs yet

'Tens of thousands will die'

William Taylor, former US ambassador to Ukraine, told Sky News he expects the UK prime minister will warn Mr Putin of the potential consequences of an invasion when they speak over the phone.

Mr Taylor said: "Undoubtedly the prime minister will make it clear to President Putin, that if he makes this terrible decision to invade Ukraine… that there will be tens of thousands of Ukrainian, military and civilians, who will die.

"There will be thousands of Russian soldiers who will die. Those kinds of actions are war crimes - and I imagine that the prime minister will point out that there is no good to come from that kind of an action."

Russia's military build-up near Ukraine has expanded to include supplies of blood that would allow it to treat casualties - another key indicator that Moscow is preparing an invasion, sources told Reuters news agency.

Labour has accused Mr Johnson of "playing catch up with other world leaders" on deterring Russian aggression against Ukraine.

Shadow defence secretary John Healey said the government is "paralysed by a mess of its own making" domestically as the PM awaits the highly anticipated "partygate" report from senior civil servant Sue Gray.

He reiterated Labour's calls for the imposition of a "register of overseas entities" to "lift the veil on who owns property and assets in the UK", after it was reported that US officials fear they will not be able to effectively sanction Mr Putin because of Russian money "entrenched" in London.

US President Joe Biden has said he was planning to move US troops to Eastern Europe and NATO countries "in the near term", adding that the number would be "not a lot".

The Pentagon has placed about 8,500 US troops on standby for possible deployment to Europe, with Mr Biden having warned his Ukrainian counterpart that Russia could take military action against the country in February.

Why is Russia worried about NATO - and what does it have to do with Ukraine?

Sir Christopher Meyer, former British ambassador to the US, said he did not have any "great hopes" about the PM's call with Mr Putin.

However, he said it was "necessary" because the UK had signed the 1994 Budapest Memorandum guaranteeing Ukraine's independence and sovereignty.

He told Sky News: "It is an obligation on us to do what we can diplomatically, and from the point of view of deterrents, to stop hot war breaking out in Ukraine because of a Russian invasion."

Sir Christopher said he did not believe Russian troops would invade Ukraine because if they did, it would "reinforce the need and necessity for an organisation like NATO, which is precisely the opposite of what Moscow wants".

Tougher sanctions expected

Mr Johnson had asked defence and security chiefs to consider further defensive military options in Europe during a high-level intelligence briefing.

This weekend, he will consider a range of options to alleviate Russian aggression in the region, including further deployments and bolstering NATO's defences.

Tougher sanctions are expected to be announced by the Foreign Office on Monday, allowing the UK to target Russia's strategic and financial interests.

The foreign secretary herself, Liz Truss, is said to have "agreed a date" to visit Moscow.

Russia's foreign minister said Ms Truss, who has previously declassified intelligence that suggested Mr Putin was plotting to install a pro-Moscow leader as head of Ukraine's government, would visit in the next two weeks.

The Britons fighting in the trenches against Russians after starting new lives

Ukrainian president says media gives impression country is already at war

It comes after Ukrainian President Volodymyr Zelenskyy said the media gave the impression Russia was already at war with Ukraine.

"There are no tanks in the streets," he told reporters on Friday.

"But media give the impression, if one is not here, that we have a war, that we have an army in the streets… That's not the case. We don't need this panic."

Sky's international affairs editor, Dominic Waghorn, said the Ukrainian government was worried about the impact of the "panic" on the country's economy.

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