An ethnic Armenian man walks past a destroyed house during fighting in Martakert province in the separatist region of Nagorno-Karabakh, Azerbaijan on April 4. (Vahan Stepanyan/AP)

Amid strident calls from world leaders for restraint, Azerbaijan and the Armenian-backed breakaway republic of Nagorno-Karabakh on Tuesday announced an immediate cease-fire to halt the worst outbreak of violence over the disputed territory in decades.

At least 50 people have died since heavy fighting resumed Saturday on the front lines of one of the former Soviet Union’s most intractable ethnic conflicts, pitting Christian Armenians against Muslim Azerbaijanis. The cease-fire was declared at noon Tuesday by officials from Azerbaijan and from the unrecognized republic of Nagorno-Karabakh, an ethnic Armenian enclave that is formally a part of Azerbaijan but has its own government with military and financial support from the Armenian government.

News reports from the front lines on Tuesday said that the fighting, which included tanks, heavy artillery and aircraft at its peak, had indeed died down. But it was unclear how long the temporary halt in hostilities would hold, as officials from the United States, France and Russia planned trips to the region this week to mediate between the sides in the conflict.

Backed by Armenia, Nagorno-Karabakh declared independence from Azerbaijan as the Soviet Union collapsed, and the bloody war that followed left about 30,000 dead and an unclear future for the territory. A cease-fire was signed in 1994, but years of negotiations under the Organization for Security and Cooperation in Europe (OSCE) have failed to yield a long-term political solution to the conflict. Sporadic fighting has continued along the “line of contact” between the two sides, though never with the ferocity seen this week.

“Everyone understands that the status quo is not only unacceptable but also unsustainable, and it cannot last for another two decades like it has been,” Elin Suleymanov, Azerbaijan’s ambassador to the United States, said in a telephone interview. Azerbaijan’s government is frustrated with the slow pace of the negotiations for a political settlement, he said, and wants to see a complete withdrawal of ethnic Armenian forces from Nagorno-Karabakh.

The cause of this week’s violence is disputed. Officials from Armenia and the Nagorno-
Karabakh territory said that Azerbaijan launched a broad offensive on Saturday, prompting a counterattack. Suleymanov said Azerbaijani troops were forced to seize strategic heights in the disputed territory to protect themselves from shelling attacks. In coming negotiations, he said, it would be an “absurd proposition” and a “major non-starter” for Azerbaijan to cede control of that land.

As both sides announced a break in the fighting, U.S., French and Russian officials prepared a series of high-level visits to Azerbaijan, Armenia and Nagorno-Karabakh to seek a more stable peace. The three countries are the co-chairs of the OSCE Minsk Group, which is tasked with mediating peace talks between the sides.

Richard Giragosian, director of the Regional Studies Center, an independent think tank in Armenia, called this week’s fighting a “near-death experience” for the cease-fire line, adding that there are no assurances that the cessation of hostilities will hold through the flurry of diplomatic activity.

“There is very little in terms of deterrence or even leverage to prevent a repeat of renewed hostilities,” said Giragosian, noting that the cease-fire does not have “external security backing or any real supervision.”

Early Tuesday evening, the ­Nagorno-Karabakh government claimed that Azerbaijani artillery had violated the cease-fire several times during the day and that its own forces had not returned fire. It was not immediately possible to verify that report.

People march with an Armenian flag during a rally Tuesday in memory of those killed during recent fightings in Nagorno-Karabakh’s main city of Stepanakert. (Staff/Reuters)

Russia has begun attempts at shuttle diplomacy. On Tuesday evening, Russian President Vladimir Putin spoke with Azerbaijani President Ilham Aliyev and Armenian President Serge Sarkisian, urging “both sides as a matter of urgency to ensure the full cessation of hostilities and compliance with a cease-fire regime,” according to a Kremlin statement. Russian Prime Minister Dmitry Medvedev will travel to Yerevan, the capital of Armenia, on Thursday, while Foreign Minister Sergei Lavrov will travel to Azerbaijan.

A wider conflict in Nagorno-Karabakh is seen as having the potential to drag in Russia, which shares a defense alliance with Armenia, and NATO member Turkey, which has said it will back the Azerbaijani government in the clashes. Tensions have soared between the two countries since Turkey downed a Russian jet over the Syrian border last year, an incident in which the pilot and a marine sent on a rescue mission were killed. Russia has accused Turkey of supporting the Islamic State and other terrorist groups and has imposed sanctions against Turkish goods and travel to Turkish resorts. On Tuesday, Turkish Foreign Minister Mevlut Cavusoglu called for a peaceful resolution to the conflict that maintains Azerbaijan’s territorial integrity.

Both sides have claimed to have killed hundreds of opposing soldiers, while reporting losses of their own in the dozens. The first day of fighting was the bloodiest, with Azerbaijan reporting 12 soldiers killed and Karabakh officials reporting 18 dead. A 12-year-old boy was also reported killed during the fighting. On Tuesday, Azerbaijan said that 16 soldiers had been killed in the last two days of fighting before the cease-fire was announced.

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Today's coverage from Post correspondents around the world