Iran’s Revolutionary Guards Corps (IRGC) seized a South Korean-flagged tanker in Gulf waters, Iranian media said on Monday, at a time of tension between Tehran and Seoul over Iranian funds frozen at South Korean banks due to US sanctions.

Several Iranian media outlets 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 tons 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. 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.

The Iranian Foreign Ministry said on 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 US sanctions.

The South Korean-flagged tanker bound for the United Arab Emirates had instead gone into Iranian territorial waters.

Satellite data from MarineTraffic.com showed the MT Hankuk Chemi off Bandar Abbas on Monday afternoon without explanation. It had been traveling from Saudi Arabia to Fujairah in the United Arab Emirates.

The ship’s owners could not be immediately reached.

The United Kingdom Marine Trade Operations, an information exchange overseen by the British royal navy in the region, acknowledged an “interaction” between a merchant vessel and Iranian authorities.

As a result, the UKMTO said the merchant vessel made an “alteration of course” north into Iran’s territorial waters.Cmdr. Rebecca Rebarich, a spokeswoman for the US Navy’s 5th Fleet, said authorities there were aware and monitoring the situation.