[WATCH] 'Devastating' consequences to Ukraine invasion: UN refugee chief

"We are gravely concerned about the fast-deteriorating situation and ongoing military action in Ukraine," Felippo Grandi said in a statement.

"The humanitarian consequences on civilian populations will be devastating. There are no winners in war but countless lives will be torn apart."

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His warning came hours after Russian President Vladimir Putin launched a full-scale invasion of Ukraine on Thursday, killing dozens and forcing hundreds to flee for their lives in the pro-Western neighbour.

"Civilian lives and civilian infrastructure must be protected and safeguarded at all times, in line with International Humanitarian Law," Grandi said.

The UN High Commissioner for Refugees said his agency and its partners in Ukraine stood "ready to provide humanitarian assistance wherever necessary and possible."

"To that effect, security and access for humanitarian efforts must be guaranteed," he said, adding that the agency had stepped up its operations in Ukraine and in the region to respond to the anticipated worsening situation.

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"UNHCR is also working with governments in neighbouring countries, calling on them to keep borders open to those seeking safety and protection," he said.

"We stand ready to support efforts by all to respond to any situation of forced displacement."