UN Secretary-General Antonio Guterres on Sunday said he was "extremely concerned over the fresh resumption of hostilities" between Armenia and Azerbaijan in the Armenia-backed breakaway region of Nagorny Karabakh.

"The Secretary-General strongly calls on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay," Guterres's spokesman said in a statement.

Arch-foes Armenia and Azerbaijan have accused each other of initiating clashes Sunday that claimed at least 23 lives and threatened to draw in regional powers Russia and Turkey.

The worst clashes since 2016 have raised the specter of a fresh war in the decades-long territorial dispute over Nagorny Karabakh.

Sixteen Armenian separatist fighters were killed and more than 100 wounded in fighting, rebel officials said.

Both sides also reported civilian casualties.

Guterres "condemns the use of force and regrets the loss of life and the toll on the civilian population," the statement said, adding that the secretary-general would speak to the president of Azerbaijan and the prime minister of Armenia.

France, Germany, Italy and the European Union also urged an immediate ceasefire, while Pope Francis prayed for peace.