Iran's Revolutionary Guards said Monday it had seized a South Korean-flagged tanker in Gulf waters for breaking "maritime environmental laws".

"A ship owned by South Korea was seized by our force's (navy) this morning," the Guards said on its website Sepahnews.

"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.

It identified the ship as the South Korean-flagged Hankuk Chemi, which it said was carrying 7,200 tonnes of "oil chemical products."

News of the seizure comes amid renewed regional tensions particularly as Iran marks the first anniversary of the assassination of its revered commander Qasem Soleimani in a US drone strike in January 2020.

Tehran on Monday also announced it had started the process to enrich uranium to 20 percent purity at its underground Fordow facility, state media reported, in a move which drew swift international concern.

The seizure of the tanker came at the request of Hormozgan Province's maritime organisation and upon the order of the provincial prosecutor, Sepahnews said.

The arrested crew were from South Korea, Indonesia, Vietnam, and Myanmar, the Guards said, without giving further details.

A photo released by the website appeared to show three speedboats and a patrol boat approaching the tanker.

The Guards' statement did not specify where was the tanker was seized or transferred to.

The British body, United Kingdom Maritime Trade Operations, said in a message that there had been "an interaction" in the Strait of Hormuz between a merchant vessel and Iranian authorities early Monday, which had caused the ship to "make an alteration of course North and ... into Iranian waters".

Fars news agency had earlier reported that the ship "has been seized in Persian Gulf waters (...) and transferred to our country's ports."

In July 2019, the Guards seized the British-flagged oil tanker Stena Impero in the sensitive Strait of Hormuz waterway for allegedly ramming a fishing boat and released it two months later.

They seized at least six other ships that year over alleged fuel smuggling.

Tensions have run high in the Gulf in recent years after the United States stepped up its campaign of "maximum pressure" on Iran following its withdrawal from the landmark nuclear deal in 2018.

The escalation saw ships mysteriously attacked, drones downed and oil tankers seized in the strategic strait -- a chokepoint through which a fifth of world oil output passes.