A blast of unclear origin killed 12 regime forces at a military airport in central Syria on Saturday, a war monitor said.
The Syrian Observatory for Human Rights monitoring group, which is based in Britain and relies on a network of sources on the ground, said it was still unclear what had caused the deadly explosion at the Shayrat airbase in Homs province.
But state news agency SANA reported that a "technical fault during the transport of expired ammunition" had killed an unspecified number of victims.
The Shayrat airbase is one of the regime's most significant installations in the centre of the country.
In 2017, US air strikes hit the base in response to a suspected sarin gas attack on the rebel-held town of Khan Sheikhun in northwest Syria that killed more than 80 people.
According to the Pentagon, US intelligence had established that the base was the launchpad for the alleged chemical attack.
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