Armenia said early on Sunday that neighbouring Azerbaijan had attacked civilian settlements in the Nagorno-Karabakh region and urged the population in the disputed region to seek refuge in shelters.

Armenia’s Defence Ministry said that its troops had downed two Azerbaijani helicopters and three drones in response to an attack it said began at 0410 GMT against civilian settlements, including the regional capital of Stepanakert.

“Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation,” the Armenian Defence Ministry said in a statement.

Azerbaijan’s Defence Ministry, in turn, said it had launched a military operation along the “contact line,” a heavily-mined no-man’s-land that separates the Armenian-backed forces from Azeri troops in the region, Russian news agencies reported.

The ministry said that an Azerbaijani helicopter had been downed but that its crew had survived.

The two former Soviet countries have long been in conflict over Azerbaijan’s breakaway, mainly ethnic-Armenian region of Nagorno-Karabakh, and border clashes have intensified in recent months.

Armenia’s Foreign Ministry condemned what it called the “aggression of the military-political leadership of Azerbaijan” and said the Armenian side would deliver an appropriate military and political response.

Ethnic Armenians in Nagorno-Karabakh declared independence during a conflict that broke out as the Soviet Union collapsed in 1991.

Though a ceasefire was agreed in 1994, Azerbaijan and Armenia frequently accuse each other of attacks around Nagorno-Karabakh and along the separate Azeri-Armenian frontier.

Reporting by Nvard Hovhannisyan; Writing by Gabrielle Tétrault-Farber; Editing by William Mallard and Stephen Coates