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

Some 28,000 people —about 23% of the region's population— have fled across the border since Azerbaijan defeated separatists who have governed the breakaway region for about 30 years in a swift military operation last week, according to Armenia's government.

Residents of Nagorno-Karabakh scrambled to flee as soon as Azerbaijan lifted a 10-month blockade on the region's only road to Armenia. That blockade had caused severe shortages of food, medicine and fuel. While Azerbaijan has pledged to respect the rights of Armenians, many residents feared reprisals.

"I think we're going to see the vast majority of people in Karabakh leaving for Armenia," said Thomas de Waal, a senior fellow at the Carnegie Europe think tank. "They are being told to integrate into Azerbaijan, a country that they've never been part of, and most of them don't even speak the language and are being told to dismantle their local institutions. That's an offer that most people in Karabakh will not accept."

The explosion took place as people lined up to fill their cars at a gas station outside Stepanakert, the region's capital, late on Monday. The separatist government's health department said that 13 bodies have been found and seven people have died of injuries from the blast. An additional 290 people have been hospitalized and scores of them remain in grave condition.

The cause of the blast remains unclear, but Nagorno-Karabakh presidential aide David Babayan said initial information suggested that it resulted from negligence, adding that sabotage was unlikely.

Armenia's health ministry said a helicopter brought some blast victims to Armenia on Tuesday morning, and more flights were expected. The Russian peacekeeping force in Nagorno-Karabakh also provided helicopters to carry victims to Armenia.

Armenian authorities also said that they brought 125 bodies over to Armenia from Nagorno-Karabakh for identification. The country's Health Ministry clarified that all of those were killed in the fighting last week.

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

Azerbaijan also said Tuesday that 33 tons of gasoline and 37 tons of diesel fuel were being sent into the region.

U.S. National Security Council spokesperson Adrienne Watson said in a statement that the U.S. urged humanitarian access to Nagorno-Karabakh. "The United States will continue to support those affected by the ongoing crisis as 28,000 people have crossed into Armenia from Nagorno-Karabakh," she said.

Watson said the U.S. would provide additional assistance to help local communities "provide shelter and essential supplies —such as hygiene kits, blankets, and clothing— to address the needs of those affected or displaced by violence in Nagorno-Karabakh."

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.

Gasoline has been in short supply in Stepanakert for months, and the explosion further added to the shortages, compounding anxiety among many residents about whether they will be able drive the 22 miles to the border.

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

Nagorno-Karabakh authorities asked residents to hold off on leaving in order to keep the road clear for emergency services and said buses would be provided for those who want to leave.

Nagorno-Karabakh was an autonomous region within Azerbaijan under the Soviet Union. 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, after a six-year separatist war that ended in 1994.

In another war in 2020, Azerbaijan took parts of Nagorno-Karabakh and completely reclaimed surrounding territory that it lost earlier. Under the armistice that ended the 2020 fighting, Russia deployed a peacekeeping force of about 2,000 to the region. Russia's influence in the region has waned amid its war in Ukraine, emboldening Azerbaijan and its main ally, Turkey.

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