An Armenian volunteer waits in the town of Askeran in the separatist Nagorno-Karabakh region. (Hrayr Badalyan/PAN via AP)

Even as Azerbaijan announced a unilateral cease-fire Sunday, reports of sporadic fighting between Azerbaijani and ethnic Armenian forces over the separatist enclave of Nagorno-Karabakh continued. Regional and world leaders called for an end to the worst violence in the region since a cease-fire halted a war over the territory in 1994.

Armenia and Azerbaijan, both former Soviet republics, fought a bloody ethnic war over the territory as the Soviet Union fell apart. About 20,000 people died. Formally a part of Azerbaijan, Nagorno-Karabakh is de facto controlled by a separatist government backed by Armenia and has increasingly been the site of sporadic border conflicts in recent years. Today, nearly all of its population is ethnically Armenian.

This weekend’s violence has been on a previously unseen scale, analysts said, with reports of the use of helicopters, drones, tanks and artillery along the “line of conflict” that separates the two sides. Thirty soldiers and a boy were killed in fighting Saturday, as Azerbaijan claimed to have seized several strategic heights and several villages from the Nagorno-Karabakh government. Both sides blamed the other for the violence.

The United States and Russia have called for an immediate end to the fighting. Turkish President Recep Tayyip Erdogan on Sunday said that he will stand with Azerbaijan “to the end” and that “we pray our Azerbaijani brothers will prevail in these clashes,” his office reported.

On Sunday, paramilitary forces from Nagorno-Karabakh said they launched a counteroffensive, which Azerbaijan claimed it had repelled, destroying 10 tanks in the process. It was not possible to confirm reports of casualties on Sunday from either side.

The Azerbaijani government said in a statement that it would implement a unilateral cease-fire Sunday afternoon, but it also said it would reinforce the territories it “liberated.” But representatives for the de facto government in Karabakh claimed fighting was continuing along the front lines, Radio Free Europe’s Armenian service reported. The news agency has also published video of ethnic Armenian reserve fighters mobilizing.

The violence came at the conclusion of a nuclear summit in Washington, which both Armenian President Serge Sarkisian and Azerbaijani President Ilham Aliyev attended. The two did not meet. U.S. Secretary of State John F. Kerry condemned the cease-fire violations, urging the sides “to show restraint, avoid further escalation and strictly adhere to the cease-fire.”

Thomas de Waal, a senior associate at the Carnegie Endowment for International Peace and an expert on the conflict, wrote that this weekend’s violence was “much more serious” than the customary violence that resumes each spring, as soldiers take potshots at one another across the border.

“It is more likely that one of the two parties to the conflict — and more likely the Azerbaijani side, which has a stronger interest in the resumption of hostilities — is trying to alter the situation in its favor with a limited military campaign,” he wrote.

Azerbaijan is openly exasperated with a decades-long process of negotiations under the Organization for Security and Cooperation in Europe and led by diplomats from the United States, Russia and France, to broker a resolution to the territorial dispute.

“There have been 22 years of attempts to find a peaceful solution to this conflict,” said Polad Bulbuloglu, the Azerbaijani ambassador to Moscow, during an interview Saturday on the radio station Govorit Moskva. “How long can it go on? We are ready for a peaceful solution to the question. But if this won’t be solved by peaceful means, then it will be solved by military means.”

Richard Giragosian, director of the Regional Studies Center, an independent think tank in Armenia, said in an interview that the West had little leverage over Azerbaijan in the conflict but that it did offer “an opening for Russian unilateral diplomacy,” possibly allowing Russian President Vladimir Putin the chance to assert his role as a peacemaker in the region.

On Saturday, Putin called for both sides “to immediately stop firing and exercise restraint,” his spokesman, Dmitry Peskov, told journalists.