LONDON -- At least 20 people were killed and nearly 300 were injured in an explosion on Monday night that tore through a makeshift gas station being used by ethnic Armenian refugees amid their exodus from the disputed enclave of Nagorno-Karabakh, according to the enclave’s local Armenian authorities.

Dozens of people are in a critical condition with severe burns and in urgent need of evacuation from the enclave where medical assistance was already minimal, the health ministry of the region’s unrecognised government, the Republic of Artsakh, said in a statement. It said many people were still missing following the blast.

The explosion and fire ripped through the fuel store on Monday night as hundreds of refugees were lining up for gas for their vehicles to leave Nagorno-Karabakh, according to local officials.

Thousands of ethnic Armenians have been leaving the enclave following a successful military offensive last week by Azerbaijan that defeated the local Armenian authorities and restored Azerbaijan’s rule over the region.

Over 13,500 people have crossed from Nagorno-Karabakh into Armenia since Sunday, according to a statement from Armenia’s government quoted by the Russian news agency Interfax. It’s feared the enclave’s entire population --estimated at 120,000 -- may seek to flee in the coming days.

Armenia’s prime minister on Monday said what was happening was the “ethnic cleansing” of Nagorno-Karabakh’s Armenian population.

Long traffic jams of people seeking to leave were visible on the only road out of Nagorno-Karabakh to a checkpoint in the "Lachin Corridor" that links the enclave to Armenia.

Nagorno-Karabakh has been at the center of a decades-long conflict between Armenia and Azerbaijan. Internationally recognised as Azerbaijan’s territory, the two countries fought a bloody war over the enclave amid the collapse of the Soviet Union, in which Armenia backed local ethnic Armenian separatists, who succeeded in establishing control over most of the region. Hundreds of thousands of Azerbaijani civilians were driven from the region during that war.

Azerbaijan reopened the conflict in 2020, launching a full-scale war that decisively defeated Armenia and forced it to largely abandon its claims to Nagorno-Karabakh. Russia helped broker a truce and dispatched a peacekeeping force there that remains deployed. Last week, Azerbaijan launched a new offensive that swiftly forced the Nagorno-Karabakh Armenian’s leadership to surrender.

Since then thousands of ethnic Armenians have been preparing to leave the enclave, which has been under Azerbaijani blockade for nine months, unwilling to live under Azerbaijan’s rule and fearing they will face persecution.

Western countries, including the United States, France and Germany, have expressed concern for Nagorno-Karabakh’s Armenian population and warned Azerbaijan it bears responsibility for their rights and security.

The Biden administration on Monday dispatched Samantha Power, currently administrator of USAID and a high-profile campaigner for human rights, and another senior State Department official to Armenia to meet with Prime Minister Nikol Pashinyan and express U.S. support for the country amid the crisis.

Power delivered a letter to Pashinyan from President Joe Biden in which the president expressed condolences for the loss of life in Nagorno-Karabakh and promised help on addressing humanitarian needs.

“I have asked Samantha Power, a key member of my cabinet, to personally convey to you the strong support of the United States and my Administration for Armenia’s pursuit of a dignified and durable regional peace that maintains your sovereignty, independence, territorial integrity, and democracy,” the letter read.

Pashinyan told Power the international community and Armenia had failed to prevent the “ethnic cleansing” of Nagorno-Karabakh’s Armenians.

“Unfortunately, at the moment the process of the ethnic cleansing of Armenians in Nagorno-Karabakh is continuing, it is happening right now. It’s a very tragic fact. We tried to inform the international community that this ethnic cleansing would happen, but, unfortunately, we did not manage to prevent it,” Pashinyan told Power and Yuri Kim, the State Department’s Acting Assistant Secretary for Europe and Eurasian Affairs, according to the prime minister’s press service.

Armenia and Azerbaijan were due to hold talks mediated by the European Union in Brussels on Tuesday, the first talks between the sides since Azerbaijan’s retook Nagorno-Karabakh.

Monday’s blast at the fuel station added a horrific complication to the exodus from Nagorno-Karabakh, with local authorities pleading for people to hold off leaving as the traffic-choking the roads out was preventing the evacuation of the severely injured.

Helicopters from Armenia’s capital, Yerevan, were reported to have flown to Nagorno-Karabakh to help evacuate some of the worst injured. A long line of ambulances was also filmed by Russian media crossing into the enclave.

The enclave’s Armenian health authorities said the hospitals in the enclave, already short of medicine and other equipment, were not equipped for the disaster.

Russia’s peacekeeping contingent said it was also providing medical assistance to the injured.