Meeting was instigated by Kiev after Russia ordered troops to be deployed in Donetsk and Lugansk regions of Ukraine.
The United Nations opened an emergency Security Council meeting on the deepening Ukraine crisis on Monday, after Russia recognised two breakaway regions there and ordered troops to be deployed as peacekeepers.
"We are confronted with a very, very dramatic situation," France's ambassador to the United Nations Nicolas de Riviere told reporters before entering the council chamber. France was among a handful of nations that called the emergency session at the instigation of Ukraine.
Addressing the session, US Ambassador Linda Thomas-Greenfield heaped scorn on Russian President Vladimir Putin's assertion that the troops he earlier ordered deployed to the rebel-held Donetsk and Lugansk regions of Ukraine would act in a peacekeeping role.
"He calls them peacekeepers. This is nonsense. We know what they really are," Thomas-Greenfield said.
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.
Thomas-Greenfield said the speech amounted to a "series of outrageous, false claims" that were aimed at "creating a pretext for war."
Russia, which currently holds the rotating presidency of the Council, had wanted the Security Council session to be closed but the United States insisted it be public.
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.
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."