The death toll from a powerful explosion at Iran's biggest port of Bandar Abbas has risen to at least 25 dead, with more than 700 people injured, state media reported on Sunday, as firefighters worked to fully extinguish the fire.

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

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