WASHINGTON (AFP) - The United States condemned "in the strongest terms" fierce clashes that left at least 30 Azerbaijani and Armenian soldiers dead on Saturday in a major escalation in violence over the disputed Nagorny Karabakh region.

Secretary of State John Kerry joined Russia in urging an immediate ceasefire after what Armenian President Serzh Sarkisian called the "largest-scale hostilities" since a 1994 truce ended a war in which Armenian-backed fighters seized the territory from Azerbaijan.

"We urge the sides to show restraint, avoid further escalation, and strictly adhere to the ceasefire," Kerry said in a statement.

"The unstable situation on the ground demonstrates why the sides must enter into an immediate negotiation under the auspices of the OSCE Minsk Group Co-Chairs on a comprehensive settlement of the conflict.

"We reiterate that there is no military solution to the conflict."

The Minsk Group, co-chaired by France, Russia and the United States, leads efforts by the Organisation for Security and Co-operation in Europe (OSCE) to find a solution to the conflict.

Separately, United Nations Secretary General Ban Ki-moon said he was "deeply disturbed".

"He is particularly concerned by the reported use of heavy weapons and by the large numbers of casualties, including among the civilian population," said a statement.

"The Secretary-General urges all relevant parties to put an immediate end to the fighting, fully respect the ceasefire agreement and take urgent steps to de-escalate the situation."

Ethnic Armenian separatists backed by Yerevan seized control of the mountainous Nagorny Karabakh region in the early 1990s war that claimed some 30,000 lives and the foes have never signed a peace deal despite the 1994 ceasefire.

The region is still internationally recognised as part of Azerbaijan and the two sides frequently exchange fire across the front, but the latest episode marked a surge in violence and sparked frantic appeals for peace from international powers.