At least 20 people were killed and nearly 300 injured in a powerful explosion that occurred Monday night at a fuel depot in the Nagorno-Karabakh region.

Local Armenian authorities said 290 people were hospitalized, dozens of them “still in critical condition” after the explosion at the facility near the regional capital Stepanakert.

The cause of the explosion is still unknown.

Dozens of people had lined up at the time at the facility where the explosion occurred because they had been promised fuel, according to Nagorno-Karabakh separatist authorities.

The blast came hours after the second round of talks between Azerbaijani officials and separatist representatives took place in the city of Khojaly, just north of the Nagorno-Karabakh capital. Azerbaijan’s presidential office said in a statement that the talks were held “in a constructive atmosphere” and that the discussion focused on humanitarian aid in the region and medical services.