At least 20 people were killed and nearly 300 others injured by an explosion at a crowded gas station in Nagorno-Karabakh as thousands of people rushed to flee the cross into Armenia, separatist authorities in the region said Tuesday.

More than 13,500 people — about 12% of the region’s population — have fled across the border since Azerbaijan’s swift military operation to fully reclaim the breakaway region after three decades of separatist rule, Armenia’s government said Tuesday morning.

The explosion took place as people lined up to fill their cars at a gas station outside the regional capital of Stepanakert late Monday. The separatist government’s health department said that 13 bodies have been found and seven people have died of injuries from the blast, the cause of which remains unclear.

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It added that 290 people have been hospitalized and scores of them remain in grave condition.

Armenia’s health ministry said a helicopter brought some blast victims to Armenia on Tuesday morning, and more flights were expected.

Azerbaijani presidential aide Hikmet Hajiyev said on X, formerly Twitter, that hospitals in Azerbaijan were ready to treat victims, but not if any had been taken to them. Azerbaijan has sent in burn-treatment medicine and other humanitarian aid, he said.

Gasoline has been in short supply in Nagorno-Karabakh for months, and the explosion further adds to local residents’ anxiety about whether they will be able to drive out. The Armenian border is about 22 miles from Stepanakert.

Cars bearing large loads on their roofs crowded the streets of Stepanakert, and residents stood or lay along sidewalks next to heaps of luggage.

The Azerbaijani military routed Armenian forces in a 24-hour blitz last week, forcing the separatist authorities to agree to lay down weapons and start talks on Nagorno-Karabakh’s “reintegration” into Azerbaijan.

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While Azerbaijan has pledged to respect the rights of ethnic Armenians in the region and restore supplies after a 10-month blockade, many local residents feared reprisals and decided to leave. Azerbaijan’s blockade of the only road connecting Nagorno-Karabakh to Armenia caused severe shortages of food, medicine and fuel in the region.

Moscow said that Russian peacekeepers in Nagorno-Karabakh were assisting the evacuation. Some 700 people remained in the peacekeepers’ camp there by Monday night.

Nagorno-Karabakh was an autonomous region within Azerbaijan under the Soviet Union, but separatist sentiment grew in the USSR’s dying years and then flared into war.

Nagorno-Karabakh came under the control of ethnic Armenian forces, backed by the Armenian military, in separatist fighting that ended in 1994. During the war in 2020, Azerbaijan took back parts of Nagorno-Karabakh along with surrounding territory that Armenian forces had claimed during the earlier conflict.

Under the armistice that ended the 2020 fighting, Russia deployed a peacekeeping force of about 2,000 to the region. But Armenian officials and regional authorities complained that the peacekeepers were unwilling or unable to end the blockade.