At least 20 people were killed in an explosion at a fuel depot in Azerbaijan's Nagorno-Karabakh on Monday, local Armenian authorities said.
They added 290 people were admitted to hospitals, dozens of them "still in critical condition".
Thousands of ethnic Armenians had fled the breakaway region, after their fighters were defeated in a lightning military operation by Azerbaijan last week.
Baku has promised to protect the rights of the roughly 120,000 Armenians who call Karabakh home but few accept its assurances. Armenian Prime Minister Nikol Pashinyan blamed Russia for failing to ensure Armenian security.
The mass departure took place amid confusion and fear.
Washington and some Western allies condemned the Azeri hostilities, which have changed the contours of the South Caucasus, a patchwork of ethnicities crisscrossed with oil and gas pipelines where Russia, the United States, Turkey and Iran vie for influence.
Moscow has said Armenia only had itself to blame for Azerbaijan's victory over Karabakh because it flirted with the West rather than working with Moscow and Baku for peace.
On Monday, senior U.S. officials arrived in Armenia, making the first such visit since the Karabakh Armenians were forced into a ceasefire last week.
From 1988 to 1994 about 30,000 people were killed and more than a million, mostly ethnic Azeris, displaced as the Armenians threw off nominal Azerbaijani control in what is now known as the First Karabakh War.
Azerbaijan regained territory in and around Nagorno-Karabakh in a second war in 2020 that ended with a peace deal brokered by Moscow and deployment of Russian peacekeepers.
Turkey, which backed Azerbaijan with weaponry in the 2020 conflict, said last week it supported the aims of Azerbaijan's latest military operation but played no part in it. (
