Iranian authorities have not said what caused the massive explosion at the port of Bandar Abbas on Saturday, killing at least 28 people, amid unconfirmed reports of the possible presence of a chemical used to make missile propellant.

Four people died and at least 516 were injured in an explosion at southwestern Iran's power of Badar Abbas, according to state media.

The blast occurred as Iran began a third round of nuclear talks with the United States in Oman. The cause wasn't immediately clear.