A large blast has occurred near the Iranian city of Natanz, home to the country’s major nuclear site. Iran’s military says the explosion was part of surprise drills testing their anti-aircraft units.

A loud explosion northeast of Natanz nuclear power plant was reported by multiple Iranian outlets late on Saturday. The blast, said to be accompanied by a flash of light, was heard near the town of Badrud, some 20 kilometers away from the facility.

#BREAKING Iran’s official news agency IRNA confirms the explosion, quoting local sources as saying that the explosion has happened in Badroud region, 20 km away from Natanz nuclear facility

Speaking to Fars news agency, the governor of Natanz acknowledged the incident, stating that its exact details were not known yet and promising to release them as soon as the situation becomes clearer.

Unconfirmed reports in the Iranian media have suggested the blast sound was caused by destruction of an unidentified drone by an anti-aircraft missile.

Shortly after the reports of the blast emerged, the Iranian military claimed it was a surprise test of local air defense systems, according to PressTV.

An anti-aircraft missile was fired some 20 kilometers away from the nuclear facility to test defense readiness, the outlet added, citing Natanz’s air-defense chief.

“An hour ago, one of the missile systems in Natanz was tested to assess its readiness, and there is nothing to worry about,” the commander stated, as quoted by IRNA.