EUROPE is the "closest it's been to armed conflict" in the past 30 years as war tensions escalate over the Ukraine crisis, top diplomats have warned.
With 100,000 Russian troops massed on the Ukrainian border, the US has to prepare for a possible escalation, an expert claimed.
"We're facing a crisis in European security," the US Ambassador to the Organisation for Security and Cooperation in Europe (OSCE) Michael Carpenter said in a Thursday press briefing.
"The drumbeat of war is sounding loud and the rhetoric has gotten shrill."
A top official in Poland, which borders Ukraine and Russia, has warned that World War 3 fears are growing, with rumbling tensions in Georgia, Armenia, and Moldova, as well as Ukraine.
"It seems that the risk of war in the OSCE area is now greater than ever before in the last 30 years," Polish Foreign Minister Zbigniew Rau said.
"For several weeks we have been faced with the prospect of a major military escalation in Eastern Europe."
He went on: "We should focus on a peaceful resolution of the conflict in and around Ukraine."
More than 100,000 Russian troops are deployed near the Ukrainian border, while separate battles with Moscow-back separatists are continuing to flare up.
It comes as negotiations between Nato and Russia failed to resolve the crisis, which is threatening to drag neighbouring powers in.
Despite four hours of talks, "significant differences" remain, according to Nato secretary-general Jens Stoltenberg.
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He also warned Russia would "pay a high price" if it used any force against Ukraine.
On Wednesday, Stoltenberg warned that Nato is preparing for the "real risk of a new armed conflict in Europe".
Moscow denies claims it is planning to invade the country. Vladimir Putin's government insists it needs guarantees for its own security, including a halt to any further Nato expansion and a withdrawal of forces from eastern European nations in Nato.
As the war of words continues, the Kremlin has condemned US legislation introduced by Senate Democrats this week that would impose sanctions on Putin himself if Russia invades Ukraine.
Spokesman Dmitry Peskov said the move would be "an outrageous measure that is comparable to breaking off relations," and an "extremely negative" gesture while negotiations continue.
Today, Russian Foreign Minister Sergei Lavrov branded the sanctions bill a "nervous breakdown".
The drumbeat of war is sounding loud and the rhetoric has gotten shrill
Michael Carpenter
In an interview on Russia's Channel One, he went on: "I read with amazement all these initiatives, like adults, serious politicians... And such steps do not do honour at all.
"There are even proposals to impose sanctions regardless of whether there is an attack on so-called Ukraine or not, simply because we are not removing troops from our own territory."
The failed negotiations follow Russia sending troops to Kazakhstan to quell anti-government protests against the pro-Moscow President Kassym-Jomart Tokayev.
At least 164 people have died including three children in unrest that broke out on January 2 in response to soaring fuel prices.
A Russian-led deployment of foreign soldiers helped police seize back buildings taken by the rioters, including troops from fellow former-Soviet states Armenia, Belarus, Kyrgyzstan, and Tajikistan.
Although they were described as "peacekeepers", they were reportedly given "shoot to kill orders" by Russian generals.
Meanwhile, UK aircraft carrier HMS Prince of Wales has embarked on its first voyage as the head of Nato's rapid response task force.
The 65,000-ton warship sailed from Portsmouth to join military exercises off Norway on Wednesday.
Conflict between Russia and Ukraine was triggered by a revolution in 2014 when the pro-Russian government of Viktor Yanukovych was overthrown.
Russia responded by annexing the Ukrainian region of Crimea - in a move widely condemned by the West.
This conflict then escalated with pro-Russian groups in Eastern Ukraine taking up arms against the state.
With Russian backing, the separatist forces created their own breakaway republics in Donetsk and Luhansk.
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Putin's troops then made their own military incursion into these regions to give their support to the rebels.
Russia continues to hold Crimea, claiming the region joined them willingly following a referendum, while the war in Donbass, eastern Ukraine remains at a stalemate.