At least 21 people were killed on Monday in a second day of clashes between Armenia and Azerbaijan, as new violence in the decadeslong territorial dispute sparked international calls to halt the fighting.

Fighting between the arch enemies over the mountainous region of Nagorno-Karabakh continued on Monday morning with both sides deploying heavy artillery.

The two former Soviet republics have been at loggerheads over Nagorno-Karabakh since 1988. Peace talks have been held since 1994, when a cease-fire was reached, but there have been occasional minor clashes along the borders.

On Sunday, the Armenian Defense Ministry reported fighting throughout the night, while Azerbaijan's Defense Ministry said Armenian forces were shelling the town of Terter.

The fighting claimed at least 21 lives. Both sides also reported civilian casualties.

Armenia's ambassador to Russia said on Monday that Turkey had sent around 4,000 fighters from northern Syria to Azerbaijan, Interfax news agency reported, an accusation Azerbaijan denied.

The Armenian Defense Ministry said on Sunday that it had shot down two of Azerbaijan's helicopters and three of its drones in Nagorno-Karabakh. The move was in response to Azerbaijan's attack on civilian settlements in the disputed territory, the ministry said.

Also on Sunday, Azerbaijan's Defense Ministry said it has launched a counteroffensive in the region.

Azerbaijan's state news agency, Azertac, reported that 12 Armenian air-defense missile systems were destroyed while one military helicopter from Azerbaijan was shot down though the crew survived.

Both sides said the clashes are ongoing.

They blamed each other for starting the fighting in the Caucasus Mountains, the worst flare-up in the conflict since a five-day war in 2016 in which more than 100 people died.

Nikol Pashinyan, Armenia's prime minister, said in an address to the nation that Azerbaijan had "declared war on the entire Armenian people once again" and that the "situation could go beyond the region's borders and threaten international peace and stability".

Ilham Aliyev, Azerbaijan's president, said in a televised speech on Sunday that the fighting "is a serious blow to the peace process", Interfax reported.

Fighting between Muslim Azerbaijan and majority-Christian Armenia threatened to embroil regional powers Russia and Turkey, with Pashinyan calling on global powers to prevent Ankara's involvement.

Turkish President Recep Tayyip Erdogan voiced support for Azerbaijan on Sunday, claiming that Armenia is "the biggest threat to peace and security in the region".

Russian President Vladimir Putin discussed the escalation in a phone call with Pashinyan, according to statements from the Kremlin and the Armenian prime minister's office.

The Kremlin's statement described Putin as expressing concern over the clashes and said: "It was noted that it is important now to take all necessary efforts to prevent a military escalation of the confrontation, and most importantly, to stop military operations."

Cease-fire urged

Meanwhile, many countries and international organizations urged an immediate cease-fire.

United Nations Secretary-General Antonio Guterres said he was "extremely concerned" and urged the two countries to stop fighting and return to talks.

Chinese Foreign Ministry spokesman Wang Wenbin said on Monday that China hopes Armenia and Azerbaijan will resolve their differences through dialogue and that the two sides will keep calm and practice restraint. Maintaining regional peace and stability is in the interests of all parties, he said.

French President Emmanuel Macron expressed his "deep concern" on Sunday and "strongly called for an immediate end to hostilities".

The United States' State Department said it had contacted the two countries and called on them to "use the existing direct communication links between them to avoid further escalation".

Agencies, Cui Haipei in Beijing and Xinhua contributed to this story.