Armenia and Azerbaijan have accused each other of violating a "humanitarian truce" in the Nagorno-Karabakh region.
Both countries announced the ceasefire from Saturday midnight via statements from their foreign ministries.
But an Armenian defence ministry spokeswoman said Azerbaijan had broken the agreement four minutes after it came into force. Azerbaijan later said Armenia had also broken the ceasefire.
The truce came after the sides denounced each other for attacking residential areas, as violence over the Nagorno-Karabakh region raged on.
"The Republic of Armenia and the Republic of Azerbaijan have agreed on a humanitarian truce from 18 October at 00:00 local time," the Armenian Foreign Ministry said, with the Azerbaijani Foreign Ministry confirming in an identical statement.
Azerbaijan on Saturday accused Armenia of striking its second-largest city, Ganja, with a missile that authorities said killed at least 13 civilians.
The Armenian Defence Ministry denied launching the strike and accused Azerbaijan of shelling residences in the city known as Stepanakert for the Armenians and Khankendi for the Azeris.
While authorities in both Azerbaijan and Armenia have denied targeting civilians, residential areas have increasingly come under shelling amid the hostilities that have raged for three weeks despite Russia's attempt to broker a cease-fire.
The former Soviet states of Azerbaijan and Armenia fought a bloody war over the mountainous region of Nagorno-Karabakh in the early 1990s.
Thousands were killed on both sides. Hundreds of thousands of people were displaced.
The war ended with a truce in 1994, although there has been sporadic violence since as the dispute remains unresolved.
The latest outburst of fighting over Nagorno-Karabakh began on September 27 and has involved heavy artillery, rockets and drones, killing hundreds and marking the largest escalation of hostilities between the South Caucasus neighbours in more than a quarter-century.
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Russia, which has a security pact with Armenia but also has cultivated warm ties with Azerbaijan, hosted top diplomats from both countries for more than 10 hours of talks that ended with a ceasefire deal last weekend.
But that agreement immediately frayed, with both sides blaming each other for breaching it.