UN Secretary General Antonio Guterres on Tuesday criticised Russia for breaching Ukraine's territory and said clashes in the country's east represented the world’s biggest security crisis in years.
Mr Guterres said Moscow’s recognition of the “independence” of two Ukrainian rebel regions breached UN rules and were a “violation of the territorial integrity and sovereignty of Ukraine”.
The US, Canada and other western nations began on Tuesday to impose sanctions against Russia for sending troops into the separatist-held Donetsk and Luhansk regions, amid reports of intensified shelling and explosions around the pro-Russian enclaves.
“Our world is facing the biggest global peace and security crisis in recent years,” said Mr Guterres, after cancelling a trip to Africa and rushing back to New York.
“We face a moment that I sincerely hoped would not come.”
He said the move by Moscow was “inconsistent” with UN rules and dealt a “death blow” to the Minsk Agreements of 2014 and 2015, which aimed to end fighting between Ukrainian forces and Russian-backed separatists.
Mr Guterres expressed concern at the “perversion of the concept of peacekeeping” used by Russia to describe its troops moving to eastern Ukraine.
“When troops of one country enter the territory of another country without its consent, they are not impartial peacekeepers,” he said. “They are not peacekeepers at all.”
He rejected Russian President Vladimir Putin’s claim that “genocide” was being perpetrated against Russian-speaking populations in Ukraine, saying it did not meet the internationally agreed criteria for the crime.
“It is high time to return to the path of dialogue and negotiations,” Mr Guterres said.
“We must rally and meet this challenge together for peace, and to save the people of Ukraine and beyond from the scourge of war.”
He used uncharacteristically tough language against Russia, one of the UN's most powerful members.
But Mr Guterres's spokesman, Stephane Dujarric, earlier said the office had only "limited" powers it could use against errant UN members.