NEW YORK — The UN chief is “extremely concerned” over renewed hostilities between Armenia and Azerbaijan in the Nagorno-Karabakh conflict zone that erupted on Sunday.

According to news reports, at least 16 people have been killed along the line of contact in the worst fighting between the two former Soviet Republics in four years.

At least 32 separatist soldiers from the Armenia-backed Nagorno-Karabakh region had been killed in the flare-up of violence since Sunday morning, the separatists' Defense Ministry said on Monday.

Azerbaijan has not announced any number for military losses. However, the loss of life could be high, with both sides claiming to have inflicted hundreds of casualties on the other.

“He condemns the use of force and regrets the loss of life and the toll on the civilian population,” UN Spokesperson Stéphane Dujarric said in a statement.

This latest skirmish between the two countries, which fought a war in the 1990s as the Soviet Union was dissolving, has heightened fears of instability in the South Caucasus, a region that provides crucial transit routes for gas and oil to world markets.

Both states have declared martial law and Armenia ordered the total mobilization of its military, according to media reports.

“The secretary-general strongly calls on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay,” Dujarric said, adding that the UN chief would be speaking to both the president of Azerbaijan and the prime minister of Armenia.

Back and forth

Armenia accused Azerbaijan of carrying out early morning air and artillery attacks on the disputed Nagorno-Karabakh region.

The two fought a six-year war over the region until a 1994 truce, but over the years, both countries have blamed the other for ceasefire violations in the enclave and along the border, including in July.

In recent months, more than a dozen soldiers and civilians have been killed in the struggle.

Dujarric underscored that the secretary-general reiterated his “full support” for the important role of the Organization for Security and Cooperation in Europe (OSCE) Minsk Group Co-Chairs and urged the sides to “work closely with them for an urgent resumption of dialogue without preconditions”.

What has the international reaction been?

European Council President Charles Michel tweeted that the fighting was of "serious concern." "Military action must stop, as a matter of urgency, to prevent a further escalation," Michel said.

Turkey’s ruling party spokesman Omer Celik tweeted: “We vehemently condemn Armenia’s attack on Azerbaijan. Armenia has once against committed a provocation, ignoring law.”

He said Turkey, which has close ties with the predominantly Turkic Azerbaijanis, would stand by Azerbaijan, adding: “Armenia is playing with fire and endangering regional peace.”

Russian President Vladimir Putin spoke on the phone with Armenia's prime minister and expressed "grave concern" over the hostilities, the Kremlin said in a statement. Russia has long been seen as an ally of Armenia.

Albanian Prime Minister Edi Rama, who is chairperson of the Organization for Security and Cooperation in Europe (OSCE), called for a de-escalation of the situation.

“I urge all involved to immediately return to the ceasefire before the human toll of this conflict increases any further,” Rama said. The OSCE has historically worked to broker peaceful negotiation between the countries. — UN News