Linda Thomas-Greenfield speaks during an emergency UN Security Council meeting at UN headquarters. Reuters

Addressing the council, US Ambassador Linda Thomas-Greenfield heaped scorn on Putin's assertion that the Russian troops would take on a peacekeeping role in the Donetsk and Lugansk areas.

"He calls them peacekeepers. This is nonsense. We know what they really are," Thomas-Greenfield said.

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Ukraine's ambassador Sergiy Kyslytsya insisted that his country's borders remain "unchangeable" despite Russia's actions.

Meanwhile, Russia's ambassador to the UN, Vasily Nebenzya, said Moscow is still "open to diplomacy for a diplomatic solution" — but warned against what he dubbed Ukrainian aggression.

"Allowing a new bloodbath in the Donbass is something we do not intend to do," he added, referring to the region encompassing Donetsk and Lugansk.

Putin's order has been widely seen as paving the way for an operation to deploy part of the potential invasion force he has massed on Ukraine's borders.

In a lengthy televised national address announcing his recognition of the rebel-held areas, Putin railed against Ukraine as a failed state and "puppet" of the West, repeatedly suggesting it was essentially part of Russia. 

Sergiy Kyslytsya speaks during an emergency UNSC meeting at UN headquarters on Monday. AP

Thomas-Greenfield said the speech amounted to a "series of outrageous, false claims" that were aimed at "creating a pretext for war."

Her remarks came just before a White House spokesperson told AFP that Washington on Tuesday would impose sanctions on Moscow following Putin's order.

'Critical' moment

Russia — which currently holds the rotating presidency of the Council — had wanted the session to be closed, but the United States insisted it be public.

Multiple countries had requested Monday's emergency meeting based on a letter from Ukraine that demanded its representative be able to attend.

Speaking before the UN late Monday, Kyslytsya challenged the Security Council to defy Russian intimidation, saying: "The United Nations is sick."

"It's been hit by the virus spread by the Kremlin. Will it succumb to this virus?" he said.

"It is in the hands of the membership."

The Under-Secretary-General for Political and Peacebuilding Affairs, Rosemary DiCarlo, voiced "regret" that Russian troops were ordered to eastern Ukraine.

"The next hours and days will be critical," DiCarlo said. "The risk of major conflict is real and needs to be prevented at all costs."

Putin's recognition of the separatist republics effectively buries a fragile 2015 peace plan for the conflict, and opens the door for direct Russian military involvement.

Barbara Woodward speaks during an emergency UNSC meeting on Ukraine at the UN headquarters.  AP

Moscow provided no details or date for any deployment of the "peacekeeping" forces, only saying that it "comes into force from the day it was signed."

British envoy Barbara Woodward said the council must be united in urging Russia to "de-escalate" and "respect its obligations."

"Russia has brought us to the brink. We urge Russia to step back," Woodward said, as China called for restraint by "all sides."

Geraldine Byrne Nason, Ireland's ambassador to the UN, called Russia's actions "a flagrant violation of international law," saying the "unilateral step" had "cast into doubt all the diplomatic efforts of past weeks."

Martin Kimani of Kenya pointed out that many countries were "birthed by the ending of empire" and urged against "dangerous nostalgia" for past borders, saying Russia's move "breaches the territorial integrity of Ukraine."

"Multilateralism lies on its deathbed tonight. It has been assaulted today, as it has been by other powerful states in the recent past," Kimani said.

Agence Frnace-Presse