The protests come after Russia announced mobilisation, aiming to add 300,000 troops to its forces. Video / AP
What form might the "catastrophic consequences" - in the words of US national security adviser Jake Sullivan - take, should Russia use nuclear weapons in Ukraine?
Possible responses the West could choose range from the military, including nuclear strikes, to those targeting Russia's economy.
Bob Seely, a British MP andexpert in Russian nuclear strategy, says the West's reaction would need to be finely judged as "there is a difference of perception which is important to understand".
"First, in Russia, tactical nuclear weapons use is serious, but arguably does not come with the same censure as in the West.
"Second, in recent Russian nuclear doctrine, tactical nuclear weapons were seen to be a deterrent to Western dominance in very high-tech precision non-nuclear weapons; so they were part of a usable arsenal.
"Third, tactical nuclear weapons could be seen as weapons to calm a crisis - the so-called escalate to de-escalate theory - whereby you trigger a nuclear weapon as a warning."
In the event of their use, a proportionate military response would be to target an airbase or intelligence hub in Crimea, according to William Alberque, director of strategy, technology, and arms control at the International Institute for Strategic Studies.
Equally, "a couple of strikes in Russia" plus a larger number in the Russian-held areas of Ukraine would be appropriate, he told the Daily Telegraph.
Of course, the US wouldn't want to "go it alone" and would instead look for military support from nations such as the UK and France, as in the strikes against Libya in 2011.
The target set for a Western response would most likely be radars or intelligence and surveillance systems connected to any nuclear strike. Equally vulnerable would be Russian early-warning or command and control nodes, such as headquarters.
These targets would likely be in Russia itself, rather than Belarus or Russian-held areas of Ukraine.
"They would shy away from attacking Russian satellites," Alberque says, as targeting of "space-based assets" has been discussed among the permanent five members of the UN Security Council and deemed "off limits".
Major General (retired) Rupert Jones says there are lots of different conventional responses the West could make if Putin was to order the use of nuclear weapons in Ukraine.
For example, a precision strike against a ship of the Russian navy or a targeted campaign against Moscow's Air Force would be conventional responses, albeit a direct attack by Nato.
Whatever action was taken, the aim must be to "make sure there's uncertainty" about future responses.
"They'd have to leave the Russians guessing about what that might look like," Jones says. Whatever action was taken, the aim must be to "make sure there's uncertainty" about future responses.
Russian President Vladimir Putin gestures as he addresses the nation in Moscow, Russia. Photo / AP
The critical thing would be to differentiate between "vertical" and "horizontal" escalation.
Vertical escalation, a commensurate or more powerful strike using the same military means, would involve the use of tactical nuclear weapons.
"The West wouldn't want vertical escalation, or even to match a Russian nuclear strike," the former head of Britain's Joint Force Command says.
"That could lose any support the action had in the UN and cede the international moral high ground.
"Horizontal escalation is likely what they'll be thinking. Make the Russians hurt elsewhere."
Such a response could see Russian military capabilities targeted in other regions of the world, for example in Syria, although this could risk spreading the war geographically.
More likely would be an attack elsewhere in, or directly linked to, the Russian homeland.
"Their Navy and sea routes are very important to them," Jones notes, "and vulnerable."
If the West closed down the Greenland-Iceland-UK gap, through which any Russian naval forces based in Murmansk have to navigate to reach the North Atlantic, "they would be in trouble".
A Yars intercontinental ballistic missile being launched from an air field during military drills. Photo / Russian Defence Ministry Press Service via AP
Difficult to get right
However, trying to calculate an action to have a desired effect is very difficult to get right.
When it comes to determining an escalation strategy, "you have to make sure your playbook is full".
"If it's not deep enough and you make a series of plays and end up still in the deterrence game, you are in trouble if you haven't planned further actions," Jones warns, adding that the use of tactical nuclear weapons should not be discounted.
The global economy would also be weaponised.
"We would say to India and China 'this is not what you signed up for'," Alberque says.
