Four oil tankers were attacked by explosions in the Strait of Hormuz and were warned not to enter restricted waters in Iran, the country’s military announced on Thursday.

The occurrences follow a period of improvement for shipping activities in the important water corridor following a assured drop in incidents and hopes for a possible easing of the tinder-fisted dispute between the United States and Iran.

Two supertankers suffered explosions that temporarily disabled them on Wednesday after allegedly ignoring warnings to do so, said Iran’s Revolutionary Guards Navy, reacting to Tehran’s accusation that they were sailing on an “unauthorised route” it says it warned against.

Iranian state media reported the explosions were the result of the boats encountering naval mines and featured still images claiming to depict the incidents. The reports went unreported however, and could not be independently substantiated.

Later, the Iranian army stated that two other tankers were hit by mines Thursday afternoon off Iranian waters, as they were traveling the eastern Strait of Hormuz. State television said that the boats had chosen to wipe out their radar systems and go into an area that was not supposed to be crossed by boats before the warning from the military.

The two tankers then blew up after the fire started, it said.

The Strait of Hormuz continues to be one of the most vital international vessels passages in the world and any disruption to this passage is a major concern for global energy market.

In a public announcement, the Revolutionary Guards Navy had announced attacks on tankers earlier in the US-Israeli war with Iran which began in February. More recently, Iranian authorities have said that vessels have been fired by what they described as naval mines when crossing into Iranian waters that have been deemed “unauthorised”.

The most recent incidents remind of ongoing concerns regarding commercial shipping security in the region.