By Nailia Bagirova and Nvard Hovhannisyan
BAKU/YEREVAN (Reuters) - A U.S.-backed ceasefire in Nagorno-Karabakh was in jeopardy as clashes resumed on Monday between Azerbaijan and ethnic Armenian forces 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.
Armenian Prime Minister Nikol Pashinyan wrote earlier on his Facebook page that the Armenian side "continued to adhere to the ceasefire."
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. local time (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.
Authorities in Nagorno-Karabakh denied this: the defence ministry said Azeri forces fired missiles on Armenian positions on the northeastern side on the line of contact and the foreign ministry said Azeri warplanes had violated the ceasefire.
Pompeo has since left Washington, landing on Monday in India on the first leg of a five-day Asian trip.
The OSCE Minsk Group, formed to mediate the conflict and led by France, Russia and the United States, also participated in Sunday's talks and is scheduled to meet the Armenian and Azeri foreign ministers again in Geneva on Oct. 29.
'HOW LONG CAN YOU NEGOTIATE?'
About 30,000 people were killed in a 1991-94 war over Nagorno-Karabakh, which Armenians regard as part of their historic homeland and Azeris consider to be illegally occupied land that must be returned to their control.
In Monday's address, Aliyev criticised the OSCE Minsk Group.
"For almost 30 years, the Minsk Group co-chairs have tried to reconcile Azerbaijan with the process of freezing the conflict, but we have created a new reality," he said. "We are fed up with these negotiations. How long can you negotiate?"
The office of Artak Beglaryan, Nagorno-Karabakh's human rights ombudsman, said 90,000 residents, or 60% of the enclave's population, had fled their homes for locations elsewhere in Nagorno-Karabakh or Armenia.
The ombudsman's office said one civilian was killed and two wounded in a missile strike on the village of Avetaranots on Monday. This was denied by Azerbaijan's defence ministry.
In all, 41 civilians in Nagorno-Karabakh and 974 servicemen had been killed, the ombudsman's office said. Azerbaijan says that 65 Azeri civilians have been killed and 297 wounded. It has not disclosed its military casualties.
(Reporting by Nailia Bagirova in Baku and Nvard Hovhannisyan in Yerevan, additional reporting by Margarita Antidze in Tbilisi, writing by Robin Paxton; Editing by Tomasz Janowski, William Maclean)