A massive explosion at a Syrian airbase killed at least 31 soldiers and paramilitary fighters on Saturday, according to the Britain-based Syrian Observatory for Human Rights.

Syrian official news agency SANA reported that the blast, at the Shayrat Airbase in central Homs province, was the result of a “technical error during the transport of expired ammunition,” but the report has not been confirmed.

Several high-ranking officers were reportedly among the dead.

In 2017, the United States launched 59 Tomahawk cruise missiles against the Shayrat Airbase in response to a chemical-weapons attack by pro-regime troops on the rebel-held town of Khan Shaykhun in which more than 80 people were killed.