MOSCOW — Renewed fighting between Armenia and Azerbaijan on Sunday threatened to reignite a three-decade-old conflict over the breakaway region of Nagorno-Karabakh.

It was the worst outbreak of fighting in the region since 2016, when four days of clashes left 200 dead. Each side blamed each other for the crisis Sunday; both declared martial law as tensions escalated.

At least one Azerbaijani helicopter was shot down Sunday. Armenia announced the full mobilization of its military as the situation threatened to spiral out of control.

Armenia claimed to have destroyed two helicopters, three drones and three tanks, saying it was in response to Azerbaijani aggression.

Azerbaijani officials said only one helicopter was shot down, with no loss of life. Azerbaijan said it was mounting a counteroffensive and claimed it had destroyed 12 Armenian air defense systems.

Russian Foreign Minister Sergei Lavrov was on the phone with both sides Sunday urging an end to fighting, according to a spokeswoman. Moscow has close ties with both sides.

“In view of the escalating situation around the Nagorno-Karabakh conflict, Lavrov is conducting intensive contacts in a bid to encourage the sides to cease fire and begin negotiations to stabilize the situation,” spokeswoman Maria Zakharova told reporters on Sunday.

World powers urged an end to hostilities after Armenia and Azerbaijan clashed on Sept. 27, stemming from tensions over the Nagorno-Karabakh region. (Reuters)

Josep Borrell, the European Union’s high representative for foreign affairs, said the escalation threatened regional security. He called for an “immediate cessation of hostilities, de-escalation and strict adherence to the cease-fire.”

The conflict between the two countries dates to the collapse of the Soviet Union. Nagorno-Karabakh, a region in Azerbaijan with a majority-Armenian population, broke away and declared independence, triggering a war that killed at least 20,000 and drove 1 million from their homes.

A cease-fire was declared in 1994, but the region remains volatile, with regular clashes along the border. Decades of peace talks mediated by the Organization for Security and Cooperation in Europe have failed to resolve the conflict.

Firing and skirmishes on the border are common: The International Crisis Group has reported close to 300 incidents since 2015.

Tensions flared again in July when at least 16 soldiers were killed in clashes on the front line between Armenia and Azerbaijan, known as the Line of Contact.

At the time, Azerbaijan’s Defense Ministry accused Armenia of shelling its positions at the Tovuz section of the border near Georgia. Armenia countered that Azerbaijan was conducting cross-border attacks.

Authorities in the mountainous Nagorno-Karabakh region said Sunday that Azerbaijan had shelled the capital, Stepanakert, and nearby settlements. Baku, which sees the region as its territory, accused Armenia of doing the shelling.

Artur Sargsyan, a defense official in the breakaway Nagorno-Karabakh region, said 16 of the region’s soldiers were killed in the fighting Sunday and more than 100 were wounded, Interfax news agency reported. 

Vahram Poghosyan, a spokesman for the region, said the situation on the border with Azerbaijan was now “under control,” the agency reported.

Earlier, the Azerbaijani Defense Ministry claimed to have taken control of several villages in Nagorno-Karabakh. Armenian officials rejected the claim.

Turkey, which has cultural and economic ties with Azerbaijan, has voiced strong support for the country since the July clashes and offered to upgrade its defense capabilities. Turkey held military exercises with Azerbaijan last month.

Turkish Defense Minister Hulusi Akar was quick to blame Armenia for the crisis Sunday. He warned that Armenia’s actions would “set the region on fire.”

Turkey and Armenia have no diplomatic ties.

Armenian Prime Minister Nikol Pashinyan called on the global community to prevent Turkey from intervening in the crisis. He warned it would have “catastrophic consequences” for the region.

Kareem Fahim in Istanbul contributed to this report.