A third attempt at halting weeks of fighting over Nagorno-Karabakh collapsed quickly on Monday with Armenia and Azerbaijan trading accusations of violating the U.S.-brokered ceasefire within minutes.
As fighting over the disputed region enters its second month, international mediators are scrambling to bring a stop to frontline clashes and shelling of civilian areas that have left hundreds dead.
The latest "humanitarian ceasefire" was announced by Washington on Sunday, after truces brokered by Russia and France fell apart over previous weekends.
It took less than an hour after the ceasefire was due to begin at 8:00 am (04:00 GMT) for the first accusations to be made.
Azerbaijan's foreign ministry said Armenian forces had shelled the town of Terter and nearby villages in "gross violation" of the truce.
Armenia's defence ministry said Azerbaijani forces had violated the truce with artillery fire on combat positions in various parts of the frontline.
Karabakh's rights ombudsman said a missile strike had hit a village in the region, killing one civilian and wounding two others.
Azerbaijan and Armenia have been locked in a bitter conflict over Karabakh since Armenian separatists backed by Yerevan seized control of the mountainous province in a 1990s war that left 30,000 people dead.
Karabakh's self-declared independence has not been recognized internationally, even by Armenia, and it remains a part of Azerbaijan under international law.
The current fighting broke out on September 27. Armenia and Azerbaijan accuse each other of having targeted civilians and of breaking the previous truces.
In an address to the nation, Azerbaijani President Ilham Aliyev said Armenia had "immediately violated" the ceasefire on Monday.
"I have ordered our military to show restraint and not give in to provocation," Aliyev said.
Armenian Prime Minister Nikol Pashinyan said on Twitter that "despite several provocations, the ceasefire is being maintained.
"The Armenian side will continue to strictly adhere to the ceasefire regime," he said.
After coming under heavy shelling at the start of the fighting, Nagorno-Karabakh's main city Stepanakert has been quieter in recent days.
AFP journalists in the city on Monday said the night had been calm. There were fewer sounds of fighting coming from the frontline on Monday morning than in previous days, though rounds of shelling could be heard in the distance.
More than 1,000 people have been reported dead in the fighting, mainly Armenian separatist fighters but also dozens of civilians on both sides.
Azerbaijan has not released any figures on its military casualties and the death toll is believed to be substantially higher, with Russian President Vladimir Putin saying last week that close to 5,000 people had been killed.
Russia, France and the United States are leaders of the "Minsk Group" which has failed since the 1990s to bring about a negotiated settlement to the conflict.
New talks in Geneva
This year's fighting is the heaviest since a 1994 ceasefire, raising fears that both Russia, which has a military alliance with Armenia, and Azerbaijan's ally Turkey could be further drawn into the conflict.
The latest ceasefire push came after U.S. Deputy Secretary of State Stephen Biegun met in Washington Saturday with Armenian Foreign Minister Zohrab Mnatsakanyan and Azerbaijani Foreign Minister Jeyhun Bayramov and the co-chairs of the Minsk Group.
U.S. Secretary of State Mike Pompeo tweeted that the negotiations had been "intensive".
The State Department said the Minsk co-chairs and the foreign ministers "agreed to meet again in Geneva on October 29" to seek "all steps necessary to achieve a peaceful settlement of the Nagorno-Karabakh conflict".
Azerbaijan has claimed to be making significant gains since the fighting began by retaking areas it lost in the 1990s war, in particular in a buffer zone outside Karabakh seized by the Armenians.
"Armenia has fallen to its knees in front of us on the battlefield and is now trying to achieve a ceasefire in order to reassemble its forces," Aliyev said in his address on Monday.
Armenia has admitted to suffering losses and called on volunteers to join the fighting at the front.
Its defence ministry said Monday there was heavy fighting in the southeast of Karabakh, where Azerbaijan says it has reclaimed a string of settlements and strategic areas