YEREVAN, Armenia — 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.

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.

The region lies in Azerbaijan but has been under control of ethnic Armenian forces backed by Armenia.

Reflecting the strategic importance of the city, 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.

Shushi’s position just 6 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.”

Nagorno-Karabakh Was an Autonomous Republic During the Soviet Era

In an episode that will likely add to the tensions in the region, a Russian military helicopter was shot down in Armenia near the border with Azerbaijan. Two servicemen were killed, but the Russian Defense Ministry said the aircraft was outside the conflict area.

The helicopter was downed near the border with Nakhcivan, an exclave of Azerbaijan surrounded by Armenia, Turkey and Iran. It was not clear who shot it down.

The ministry said the helicopter was accompanying a military convoy, but did not 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.

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 to determine the region’s final status faltered, and the region was separated from the rest of Azerbaijan by a demilitarized zone.

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 has not stated its losses.