At least 16 troops and civilians have been killed in the worst fighting between Azerbaijani and Armenian forces since 2016, raising fears of instability in a region that provides crucial transit routes for gas and oil to the international market. Armenia declared martial law and ordered the total mobilisation of its military on Sunday after claiming to have destroyed several Azerbaijani aircraft and tanks in early-morning clashes. It accused Azerbaijan of carrying out air and artillery attacks on a disputed region, Nagorno-Karabakh, that is internationally recognised as Azerbaijani territory but which broke away in 1991 and is run by ethnic Armenians. Azerbaijan said it had counter-attacked in response to Armenian shelling. The clashes prompted a flurry of diplomacy to prevent a 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. Human right activists in Armenia said two civilians, a woman and a child, had been killed by Azerbaijani shelling. Armenian military officials have reported at least 10 casualties on their side. Officials in Baku, the Azerbaijani capital, said an unspecified number of their civilians had been killed and six wounded, and Nagorno-Karabakh said 16 of its military staff had been killed. The reports could not be independently confirmed. Azerbaijan’s army said it had taken control of several villages in Nagorno-Karabakh as of Sunday afternoon, a claim that Armenia rejected. The long-running dispute in the south Caucasus attracts regional and western concern because the area is a corridor for pipelines carrying oil and gas from the Caspian Sea to global markets. Turkey has strong cultural and economic ties with Azerbaijan and has threatened to stand with it in any conflict. Russia, another regional power, is traditionally close to Armenia, but has been forging links with Azerbaijan’s elites in past years. The Turkish president, Recep Tayyip Erdoğan, said in a statement posted on Twitter that Armenia had “once again showed that it is the biggest threat to peace and tranquility in the region” and that Turkey stood by Azerbaijan “with all its means, as always”. Armenia’s defence ministry on Sunday said its troops had destroyed three tanks and shot down two helicopters and three unmanned aerial vehicles in response to an attack on civilian targets including Nagorno-Karabakh’s capital, Stepanakert. “Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation,” the ministry said in a statement echoed by the foreign ministry. The Armenian prime minister, Nikol Pashinyan, wrote on Twitter: “We stay strong next to our army to protect our motherland from Azeri invasion.” Azerbaijan denied the Armenian defence ministry statement, saying it had “complete advantage over the enemy on the front”, and accused Armenian forces of launching “deliberate and targeted” attacks along the front line. “We defend our territory, our cause is right,” Azerbaijan’s president, Ilham Aliyev, said in an address to the nation. The two former Soviet Republics have clashed for years over the mountainous region of Nagorno-Karabakh, a mainly ethnic Armenian territory that is officially part of Azerbaijan but which broke away from the country as the Soviet Union was dissolving. Armenia and Azerbaijan fought a six-year war over the region until a ceasefire in 1994, and since then Nagorno-Karabakh has governed itself as the de facto independent Republic of Artsakh. Both countries have continued to accuse each other of violating the ceasefire in the enclave and elsewhere along their border in the years since, including throughout 2020. More than a dozen soldiers and civilians have been killed in fighting in recent months. At least 200 people were killed in a revival of the conflict in April 2016. Reuters contributed to this report