At least 16 military and several civilians were killed on Sunday in the heaviest clashes between Armenia and Azerbaijan since 2016, reigniting concern about stability in the South Caucasus, a corridor for pipelines carrying oil and gas to world markets.

The clashes between the two former Soviet republics, which fought a war in the 1990s, were the latest flare-up of a long-running conflict over Nagorno-Karabakh, a breakaway region that is inside Azerbaijan but is run by ethnic Armenians.

Nagorno-Karabakh said 16 of its serviceman had been killed and more than 100 wounded after Azerbaijan launched an air and artillery attack early on Sunday. Armenia and Nagorno-Karabakh declared martial law and mobilised the male population.

Azerbaijan, which also declared martial law, said its forces responded to Armenian shelling and that they had seized control of up to seven villages, though Nagorno-Karabakh denied this. Azerbaijan’s prosecutor’s office said five members of one family had been killed by Armenian shelling.

The clashes prompted a flurry of diplomacy to prevent a new flare-up of a decades-old conflict between majority Christian Armenia and mainly Muslim Azerbaijan, with Russia calling for an immediate ceasefire and another regional power, Turkey, saying it would support Azerbaijan.

Pipelines shipping Caspian oil and natural gas from Azerbaijan to the world pass close to Nagorno-Karabakh. Armenia also warned about security risks in the South Caucasus in July after Azerbaijan threatened to attack Armenia’s nuclear power plant as possible retaliation.