Armenian Prime Minister Nikol Pashinyan has said it is "very important" for a ceasefire between Karabakh separatists and Azerbaijan to hold, and that he hoped it will be ensured by Russian peacekeepers in the disputed region.
Ethnic Armenians in Azerbaijan's breakaway region of Nagorno-Karabakh earlier agreed to a Russian proposal for a ceasefire.
Today's agreement comes a day after Azerbaijan began an offensive to take control of the enclave that killed dozens and injured hundreds.
The separatist Armenian forces in Karabakh said Azerbaijan had broken through their lines and seized a number of heights and strategic road junctions. He added that the other nations were not acting on the incident.
Mr Pashinyan said that the capital Yerevan did not take part in drafting a ceasefire agreement.
"Armenia did not participate in drafting the text of the ceasefire declaration in Nagorno-Karabakh under the mediation of Russian peacekeepers," Mr Pashinyan said, reiterating that Yerevan "does not have an army" in the separatist-controlled enclave.
The self-styled "Republic of Artsakh" said that in such circumstances, it had no choice but to cease hostilities from 1pm local time.
"The authorities of the Republic of Artsakh accept the proposal of the command of the Russian peacekeeping contingent to cease fire," it said.
"With the mediation of the command of the Russian peacekeeping contingent stationed in Nagorno-Karabakh, an agreement was reached on the complete cessation of hostilities from 1pm on 20 September 2023."
Azerbaijan confirmed that a ceasefire agreement had been reached.
It said Russian peacekeepers relayed the Karabakh Armenian appeal for a ceasefire to Azerbaijan.
It did not immediately set out the conditions.
Azerbaijan began its operation against Nagorno-Karabakh yesterday after some of its troops were killed in what Baku said were attacks from the mountainous region, which Azerbaijan had blockaded for nine months.
Baku had demanded that the separatist political authorities in Karabakh, which is recognised internationally as part of Azerbaijan, also disband before any talks are held about the future of the region, which Azerbaijan wants to fully integrate.
Azerbaijan and Armenia fought two wars over the mountainous territory, in the 1990s and in 2020.
Russia and the United Nations had called for an end to fighting before today's announcement.
US Secretary of State Antony Blinken spoke by phone with the leaders of both Armenia and Azerbaijan, with French President Emmanuel Macron also speaking to Armenian Prime Minister Nikol Pashinyan.
Fears of a fresh war in the volatile Caucasus region have been growing recently, with Armenia accusing Azerbaijan of a troop build-up around the disputed territory.
Separatists said Azerbaijan yesterday pounded the mountainous territory with artillery, aircraft and drones.
Blasts rocked the separatist stronghold od Stepanakert.
More than 7,000 people were evacuated from 16 villages, the separatists said.
Azerbaijan's defence ministry said it had taken control of more than 60 military positions during "localised anti-terrorist measures".
