TEHRAN: Iran said Monday its Revolutionary Guards had seized a South Korean-flagged tanker in Gulf waters for breaking "environmental laws," at a time of heightened tensions with the United States.

Seoul demanded the release of the Hankuk Chemi and its multinational crew of 20 and deployed its military's anti-piracy unit to the area close to the strategic Straits of Hormuz.

The Guards said on their Sepahnews website that the ship carrying 7,200 tonnes of oil chemical products "was seized by our force's (navy) this morning". "This tanker was headed from Saudi Arabia's Al Jubail port and was seized due to the repeated infringement of maritime environmental laws," it added.

The seizure came days before South Korea's deputy foreign minister was due to visit Tehran according to Iran's foreign ministry, as Iran wants South Korea to release billions in its assets held as a result of US sanctions.

It also follows days of high US-Iranian tensions and came on the day Iran said it had started the process to enrich uranium to 20 percent purity, a move which drew swift international concern.