The US has accused Russia of “tearing to shreds” a crucial agreement on the Ukraine conflict, with its dramatic escalation of tensions in Europe.

In a scathing takedown, the US ambassador to the United Nations, Linda Thomas-Greenfield, told a snap meeting of the UN Security Council on Tuesday that Russian President Vladimir Putin had “put before the world a choice”.

“We must meet the moment, and we must not look away. History tells us that looking the other way in the face of such hostility will be a far more costly path,” she said.

She said an “attack on Ukraine was an attack on the sovereignty of every UN member state and the UN charter”.

The emergency meeting of the 15-member council came just hours after Mr Putin said he would recognise two breakaway regions of eastern Ukraine – the self-proclaimed Donetsk People’s Republic and the Lugansk People’s Republic – and send in troops on a “peacekeeping mission”.

Ms Thomas-Greenfield said it was “nonsense” that Russian troops in the separatist eastern regions of Ukraine were peacekeepers.

“If Russia invades Ukraine even further, we will see a devastating loss of life, unimaginable suffering,” she said.

“In essence, Putin wants the world to travel back in time to time before the United Nations to a time when empires ruled the world. But the rest of the world has moved forward.

“It is not 1919, it is 2022.”

Russia, however, used the emergency meeting to blame Ukraine for the incursion. Vasily Nebenzya said that “allowing a new bloodbath in the Donbass is something we do not intend to do”.

Ukraine has strongly denied escalating violent tensions.

Mr Nebenzya said Russia remained open to diplomacy, and insisted Ukraine was still bound by the Minsk agreement, which set out steps to end the eight-year conflict in Ukraine’s east and resolve the status of the breakaway areas.

Ms Thomas-Greenfield said there was no need to guess at Mr Putin’s motivation for sending in troops.

“President Putin has torn the Minsk Agreement to shreds. We have been clear that we do not believe he will stop at that,” she said, referring to the agreements of 2014 and 2015 that aimed to end conflict between the Ukrainian army and Russian-backed separatists in eastern Ukraine.

Tensions between Moscow and Western capitals are high following weeks of US accusations that Russia has deployed up to 150,000 troops near Ukraine’s borders for an invasion.

Early on Tuesday (Australian time), Mr Putin officially recognised the two breakaway regions as independent, defying Western warnings that such a step would be illegal and wreck peace negotiations.

Mr Putin has also ordered the deployment of Russian forces to the Donbass region in eastern Ukraine “keep the peace”.

A Reuters witness has reported saw tanks and other military hardware moving through the separatist-controlled city of Donetsk after Mr Putin’s announcement.

About five tanks were seen in a column on the edge of Donetsk and two more in another part of town, a Reuters reporter said. No insignia were visible on the vehicles.

“We remain open to diplomacy for a diplomatic solution. However, allowing new bloodbath in the Donbass is something we do not intend to do,” Russia’s UN Ambassador Vassily Nebenzia told the security council.

China’s UN Ambassador Zhang Jun said “all parties concerned must exercise restraint, and avoid any action that may fuel tensions”. He said Beijing welcomed and encouraged every effort for a diplomatic solution.

Ukraine called for the security council to meet after Russia’s recognition of the breakaway regions on Monday. Eight council members, including the US, Britain and France, backed Kyiv’s request.

It was the third security council meeting on Ukraine in as many weeks. The body has met dozens of times to discuss the Ukraine crisis since Russia annexed the Crimea region in 2014. It cannot take any action because Russia is a veto-power, along with France, Britain, China and the US.

President addresses Ukraine

Ukrainian President Volodymyr Zelensky gave a televised address to the nation early on Tuesday, vowing the nation would “not give away anything to anyone”.

“We are on our land, we are not afraid of anything and anyone, we don’t owe anything to anyone, and we will not give away anything to anyone. And we are confident of this,” Mr Zelensky said, adding said the move was a violation of Ukraine’s “national integrity and sovereignty.”

He said Ukraine’s international borders would “remain as such”, despite Russia’s “declarations and threats”.

The country has also called for “clear and effective steps” from its international supporters.

Mr Zelensky said Ukraine had initiated an emergency meeting with the Normandy Four – a group made up of Germany, Russia, Ukraine and France.

He warned that Mr Putin’s move undermined “peaceful” negotiations and “may mean a one-sided exit of Russian Federation out of the Minsk Agreement and ignoring of Normandy agreement.”

The country’s foreign minister, Dmytro Kuleba, said he had spoken to US Secretary of State Antony Blinken ahead of a meeting in Washington.

“Key topic: sanctions. I underscored the need to impose tough sanctions on Russia in response to its illegal actions,” he tweeted on Tuesday.

CNN reported that the US would announce new sanctions against Russia within hours.

“We are coordinating with allies and partners on that announcement,” a spokesperson said.

In Britain, Prime Minister Boris Johnson will chair a meeting of the government’s emergency committee on Tuesday morning to determine what Downing Street called “a significant package of sanctions” against Russia.

The presidents of the European Council and Commission, Charles Michel and Ursula von der Leyen, have condemned what they called Russia’s “illegal act” and promised European sanctions.

Elsewhere, the US has ordered its diplomatic staff to leave Ukraine.

-with AAP