WASHINGTON (Reuters) - U.S. Deputy Secretary of State Stephen Biegun spoke separately to the foreign ministers of Azerbaijan and Armenia on Monday and urged an immediate ceasefire in the Nagorno-Karabakh region, the State Department said. In his conversations with Azeri Foreign Minister Jeyhun Bayramov and Zohrab Mnatsakanyan of Armenia, Biegun expressed “deep concern over reports of the escalation of military action and expanding theater of operations in the Nagorno-Karabakh conflict,” the department said in a statement. “Deputy Secretary Biegun urged the sides to agree to a ceasefire immediately and resume negotiations under the auspices of the Minsk Group Co-Chairs to find a durable resolution to the conflict,” it said. “The deputy secretary stressed to the Foreign Ministers that there is no military solution to the Nagorno-Karabakh conflict.” Earlier, NATO chief Jens Stoltenberg added his voice to calls for an immediate end to the clashes over Nagorno-Karabakh, a mountain enclave that belongs to Azerbaijan under international law but is populated and governed by ethnic Armenians. Azerbaijan and Armenia accused each other on Monday of attacking civilian areas and said the death toll was rising from the deadliest fighting in the South Caucasus for more than 25 years. Prospects for a ceasefire appeared remote after fighting intensified at the weekend, with hundreds killed in clashes involving artillery, tanks and fighter planes since Sept. 27. The Minsk Group Biegun’s statement referred to was set up by the Organization for Security and Co-operation in Europe (OSCE) in the early 1990s to encourage a resolution to the conflict. It is co-chaired by France, Russia and the United States.