Gulf of Oman
Iran rarely reports on mishaps during its exercises, signalling the severity of the incident.
The move follows recent attacks on oil tankers in the Gulf of Oman.
The US have accused Iran of attacking two oil tankers near the strait of Hormuz. The US military's Central Command released a video it says shows Iranian forces removing an unexploded mine from one of the ships before the attack. Iran has denied involvement amid heightened tensions between the two countries.