Seoul has dispatched its anti-piracy unit to the Gulf after Iran seized a South Korean-flagged oil tanker for breaking maritime environmental laws, the defence ministry said Monday.

"The defence ministry immediately dispatched the Cheonghae unit to waters near the Strait of Hormuz shortly after receiving a report on the situation of Iran's seizure of our commercial vessel," the ministry said in a statement.

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

It identified the ship as the Hankuk Chemi, which it said 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