Clashes have erupted between Armenian and Azerbaijani forces in the disputed Nagorno-Karabakh region, with civilian deaths reported by both sides.
Armenia said Azerbaijan had launched an air and artillery attack. It later declared martial law and total military mobilisation.
Azerbaijan blamed Armenia and said it was responding to shelling along the whole front.
The long-running conflict has flared up again in recent months.
Turkey’s President Recep Tayyip Erdogan has pledged support for Azerbaijan, calling Armenia “the biggest threat to peace and tranquillity in the region.”
Russia’s Foreign Ministry, meanwhile, called for an immediate ceasefire and talks to stabilise the situation.
Both Armenia and Azerbaijan were part of the Soviet Union before its collapse in 1991.
For four decades they have been stuck in an unresolved conflict over Nagorno-Karabakh, internationally recognised as part of Azerbaijan but controlled by ethnic Armenians.
Turkey has close ties to Azerbaijan and does not have relations with Armenia because of a dispute over the mass killing of Armenians during the Ottoman era. Armenia says this was a genocide, but Turkey staunchly denies this.
What are the two sides saying?
The Armenian Defence Ministry said an attack on civilian settlements, including the regional capital Stepanakert, began at 08:10 local time (04:10 GMT).
It said it had shot down two helicopters and three drones, and destroyed three tanks.
“Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation,” it said in a statement.
Officials said a woman and child had been killed, and further reports of casualties were being verified.
“Get ready to defend our sacred homeland,” Prime Minister Nikol Pashinyan said in a statement. He earlier accused Azerbaijan of “pre-planned aggression”.
Meanwhile Azerbaijan blamed Armenia for starting the fighting.
Intensive shelling of several villages had led to civilians being killed or wounded, and severe damage to infrastructure, its defence ministry said.
The country announced a “counter-offensive operation of our troops along the entire front to suppress the combat activity of the armed forces of Armenia and ensure the safety of the civilian population”.
It added that one helicopter had been lost but the crew had survived, and said 12 Armenian air defence systems had been destroyed. It denied other losses reported by Armenia.
Later on Sunday, a defence ministry spokesperson said several villages “which were under enemy occupation for many years, have been liberated”. The claim was rebuffed by Armenia’s defence ministry spokesperson, who said it was “not consistent with the reality.”
Nagorno-Karabakh – key facts
- A mountainous region of about 4,400 sq km (1,700 sq miles)
- Traditionally inhabited by Christian Armenians and Muslim Turks
- In Soviet times, it became an autonomous region within the republic of Azerbaijan
- Internationally recognised as part of Azerbaijan, but majority of population is ethnic Armenian
- An estimated one million people displaced by 1990s war, and about 30,000 killed
- Separatist forces captured some extra territory around the enclave in Azerbaijan in the 1990s war
- Stalemate has largely prevailed since a 1994 ceasefire
- Russia has traditionally supported the Armenians
In a TV address, Azerbaijani President Ilham Aliyev said Armenia’s policy was “a new war for new territories”.
“Armenia has been consciously provoking Azerbaijan, and they will see the bitter results of this,” he said.
“Armenia is an occupying country, and an end must be put to this occupation and an end will be put to it.”
The Organisation for Security and Co-operation in Europe (OSCE) has long been trying to mediate a settlement of the conflict, with diplomats from France, Russia and the US – making up the OSCE Minsk Group – trying to build on a ceasefire accord signed in 1994.