MUSCAT, Oman, April 26, (AP): A massive explosion and fire rocked a port Saturday in southern Iran purportedly linked to a shipment of a chemical ingredient used to make missile propellant, killing four people and injuring more than 500 others.

The blast at the Shahid Rajaei port happened as Iran and the United States met Saturday in Oman for the third round of negotiations over Tehran’s rapidly advancing nuclear program.

While no one in Iran outright suggested that the explosion came from an attack, even Iranian Foreign Minister Abbas Araghchi, who is leading the talks, on Wednesday acknowledged that "our security services are on high alert given past instances of attempted sabotage and assassination operations designed to provoke a legitimate response.”

For hours, authorities in Iran offered no clear explanation for what caused the blast at the port, which is just outside of Bandar Abbas, though they did deny that the explosion had anything to do with the country's oil industry.

However, the port took in a shipment of "sodium perchlorate rocket fuel” in March, the private security firm Ambrey said. The fuel is part of a shipment from China by two vessels to Iran first reported in January by the Financial Times.

The fuel was going to be used to replenish Iran's missile stocks, which had been depleted by its direct attacks on Israel during the war with Hamas in the Gaza Strip. "The fire was reportedly the result of improper handling of a shipment of solid fuel intended for use in Iranian ballistic missiles," Ambrey said.