The Russia-brokered truce, put in place after marathon talks in Moscow between both sides, was meant to halt fighting to allow ethnic Armenian forces in Nagorno-Karabakh and Azeri forces to swap prisoners and war dead. Decades of dispute followed and in the late 1980s, as the iron grip of the Soviet Union began to loosen, Nagorno-Karabakh's regional parliament voted to become part of Armenia.
If the truce had held, it would mark a major diplomatic coup for Russian Federation, which has a security pact with Armenia but also cultivated ties with Azerbaijan.
However, minutes after the cease-fire took force, both sides accused each other of continuing attacks in violation of the deal.
Russian Foreign Minister Sergei Lavrov said on Monday that no changes were envisaged to the format for peace talks over Nagorno-Karabakh after Azerbaijan proposed that its ally Turkey should be involved.
Mr Lavrov said he believed the "all-night vigil" that clinched the ceasefire would "not be in vain" and that the issue could be resolved on the ground "in the nearest time".
French foreign ministry spokeswoman Agnes von der Muhll said it was "indispensable and urgent" that the truce was "strictly" obeyed.
A woman sits in ruins of her house in Terter, Azerbaijan.
"They wake up and cry".
Armenian military officials also said that Nagorno-Karabakh forces shot down an Azerbaijani Su-25 warplane, a claim that Azerbaijan denied.
"Based on the information about the recruitment and use of foreign mercenaries by Azerbaijan since the first days of the aggressive war, the Armenian central military prosecutor's office has launched a criminal case into global terrorism", the Prosecutor General's Office said in a press release.
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"During the conversation, the sides exchanged views on the situation in the Nagorno-Karabakh conflict zone developing after the trilateral talks in Moscow on October 9-10", it said.
Azerbaijan said 41 Azeri civilians had been killed and 207 wounded since September 27.
Inhabited mostly by ethnic Armenians, Nagorno-Karabakh seceded from Azerbaijan in 1991 and declared itself an independent republic after the 1994 armistice.
The EU foreign affairs chief pushed the sides "to engage in substantive negotiations without delay under the auspices of the so-called "Minsk Group".
Mnatsakanyan was due to meet the Minsk Group co-chairs in Moscow after his discussions with Lavrov.
Azerbaijan's President Ilham Aliyev, Turkey's Foreign Minister Mevlut Cavusoglu and officials attend a meeting in Baku, Azerbaijan, October 6, 2020.
Its origins date back to when both Armenia and Azerbaijan were Soviet republics.
Russian Federation has a military base in Armenia and both are members of the Collective Security Treaty Organization (CSTO) alliance.
On Monday morning, the Azerbaijani Defense Ministry stated that the Armenian forces launched attacks on sites in the direction of Agdera, Agdam, Fazuli and Gabriel.
Moscow had repeatedly urged a ceasefire and a political settlement of the differences.