DUBAI/LONDON—Iran’s Revolutionary Guards Corps seized a South Korean-flagged tanker in Gulf waters, Iranian media said on Jan. 4, at a time of tension between Tehran and Seoul over Iranian funds frozen at South Korean banks due to U.S. sanctions.

Several Iranian media outlets, including Iranian state TV, said the Guards navy captured the vessel for polluting the Gulf with chemicals. The semi-official Tasnim news agency published pictures showing what it identified as Guards speed boats escorting the tanker Hankuk Chemi, which it said was carrying 7,200 tonnes of ethanol.

It said the vessel’s crew members had been detained and included nationals of South Korea, Indonesia, Vietnam and Myanmar, but did not say how many there were. It said the tanker was being held at Iran’s Bandar Abbas port city.

South Korea’s foreign ministry did not have an immediate comment. The U.S. Navy’s Bahrain-based Fifth Fleet was aware and monitoring the situation, spokeswoman Rebecca Rebarich said in response to a Reuters query.

Iranian authorities have yet to comment on the incident, which comes ahead of an expected visit by South Korea’s deputy foreign minister to Tehran.

Iranian foreign ministry said on Jan. 4 that the visit would happen in coming days and would discuss Iran’s demand that South Korea release $7 billion in funds frozen in South Korean banks because of U.S. sanctions.

The United States re-imposed sanctions on Iran in 2018 after Washington withdrew from Iran’s 2015 nuclear deal with six major powers. Tehran calls the U.S. sanctions economic warfare.

Two maritime security companies earlier said the Hankuk Chemi appeared to have been seized by Iranian authorities.

British firm Ambrey said the South Korean-flagged vessel, owned by DM Shipping Co, had departed from the Petroleum Chemical Quay in Jubail, in Saudi Arabia, before the incident and had since been tracked inside Iranian territorial waters headed toward Bandar Abbas.

Another maritime security firm, Dryad Global, said on its website the chemical tanker had “likely been detained by Iranian forces” in the Strait of Hormuz while inbound to Fujairah in the United Arab Emirates.

In early 2019, Iran heightened tensions in the world’s busiest oil waterway by seizing British-flagged tanker Stena Impero two weeks after a British warship had intercepted an Iranian tanker off the coast of Gibraltar.