South Korea's foreign ministry on Monday demanded the "early release" of an oil tanker seized by Iran in Gulf waters for breaking maritime environmental laws.

"The foreign ministry... is demanding an early release of the vessel," it said in a statement, adding that it had confirmed the safety of the tanker's crew.

Seoul dispatched its anti-piracy unit to the Gulf immediately after receiving reports the ship had been seized, according to the defence ministry.

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.

Iran's Revolutionary Guards said it had seized the South Korean-flagged Hankuk Chemi in Gulf waters "due to the repeated infringement of maritime environmental laws".

It said the vessel was carrying 7,200 tonnes of "oil chemical products."

The arrested crew were from South Korea, Indonesia, Vietnam, and Myanmar, the Guards said on its website Sepahnews, 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 the tanker was seized or transferred to.

sh/rma