BAKU/YEREVAN (Reuters) - A U.S.-backed ceasefire in Nagorno-Karabakh was in jeopardy on Monday as Azerbaijan and ethnic Armenian forces renewed fighting in the mountain enclave, defying international efforts to end a conflict that has killed hundreds in the last month.Azeri President Ilham Aliyev said in a televised address that he wanted to resolve the conflict “by political and military means” after both sides accused each other of breaking a truce agreed hours earlier in Washington. Speaking live on Facebook later on Monday, Armenian Prime Minister Nikol Pashinyan said he did not believe Azerbaijan was interested in a peaceful resolution to the conflict. “The Armenian people are ready for mutual concessions, even painful ones, but not for the capitulation of Karabakh,” he said. The latest fighting in Nagorno-Karabakh, a mountainous part of Azerbaijan populated and controlled by ethnic Armenians, erupted on Sept. 27 and is the worst in the South Caucasus since the 1990s. Two Russian-brokered ceasefires have failed to hold. World powers want to prevent a wider war that might draw in Turkey, which voices strong support for Azerbaijan, and Russia, which has a defence pact with Armenia. The conflict, close to pipelines that carry Azeri oil and gas to international markets, has also strained relations between Ankara and its NATO allies. A third ceasefire since Oct. 10 was agreed on Sunday after separate talks in Washington between U.S. Secretary of State Mike Pompeo and the foreign ministers of Armenia and Azerbaijan. Within minutes of its coming into force at 8 a.m. (0400 GMT), Azerbaijan’s defence ministry said in a statement that Armenian forces had shelled villages in the Terter and Lachin regions, located at opposite ends of the conflict zone.