YEREVAN, Armenia (AP) — Azerbaijani forces took control of a strategic city in the separatist territory of Nagorno-Karabakh and were nearing its capital, a spokesman for the region’s government confirmed Monday, and Azerbaijan said it shot down a Russian helicopter over Armenia far from the fighting. Two Russian servicemen were killed.

The seizure of Shushi — which Azerbaijani President Ilham Aliyev had claimed a day earlier — is the most significant military development since fighting between Azerbaijani and Armenian forces over Nagorno-Karabakh restarted in September.

Separately, Azerbaijan said that its forces shot down the Russian military helicopter as it flew over Armenia. The incident happened about 70 kilometers (45 miles) away from Nagorno-Karabakh, but Azerbaijan said the war there was a contributing factor.

Nagorno-Karabakh lies in Azerbaijan but has been under control of ethnic Armenian forces backed by Armenia.

Shushi’s position just six miles (10 kilometers) from the regional capital of Stepanakert gives strategic advantage to whoever holds it. The city also lies along the main road connecting Stepanakert with Armenia. Long lines of vehicles jammed the territory’s main road on Sunday as Nagorno-Karabakh residents fled the fighting into Armenia.

“Unfortunately, we are forced to admit that a series of failures still haunt us, and the city of Shushi is completely out of our control,” Vagram Pogosian, a spokesman for the president of the government in Nagorno-Karabakh, said in a statement on Facebook. “The enemy is on the outskirts of Stepanakert.”

Armenian Prime Minister Nikol Pashinian insisted that “the fight for Shushi is continuing,” indicating Armenian forces were either trying to retake the city or prevent advances toward the capital.

The Russian helicopter was downed near the border with Nakhcivan, an exclave of Azerbaijan surrounded by Armenia, Turkey and Iran.

Russia's Defense Ministry said the helicopter was accompanying a military convoy, but didn't say what the purpose of the convoy was. Russia maintains a large military base in northern Armenia, about 90 miles (150 kilometers) from the site where the helicopter was downed.

A statement from Azerbaijan's Foreign Ministry noted that the helicopter was flying in the dark at low altitude and “in the context of these factors and in light of the tense situation in the region and increased combat readiness in connection with possible provocations of the Armenian side, the duty combat crew decided to open fire to kill.”

Nagorno-Karabakh was an autonomous republic during the Soviet era. As the Soviet Union began to break apart, there were clashes and then full-scale war erupted after the Soviet collapse in 1991. An estimated 30,000 people died in the fighting, which ended with a 1994 truce.

Since then, international mediation efforts by the Organization for Security and Cooperation in Europe's “Minsk Group” to determine the region’s final status faltered, and the region was separated from the rest of Azerbaijan by a demilitarized zone.

Aliyev on Monday urged U.S. President-elect Joe Biden to intensify mediation efforts. In a congratulatory letter to Biden on his election victory, Aliev said “Azerbaijan expects the United States and other OSCE Minsk Group co-chairs to step up their efforts to find a just solution to the conflict.”

The current fighting has continued despite the declaration of several cease-fires. Armenia says more than 1,200 Armenian troops have been killed in the war. Azerbaijan hasn't stated its losses.

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Associated Press writers Jim Heintz in Moscow, and Aida Sultanova in London, contributed to this story.