Iranian forces captured the South Korean-flagged chemicals tanker in the Gulf's Strait of Hormuz. Tehran wants US sanction-related funds frozen in South Korean banks released.

Revolutionary Guards in speed boats seized the Hankuk Chemi, a tanker carrying ethanol, in the Straits of Hormuz as it was traveling from Saudi Arabia to the United Arab Emirates, Iranian media outlets reported Monday. 

Maritime tracking firms said the ship ended up stationary in Iranian territorial waters off Iran's Bandar Abbas port city.

South Korea's Foreign Ministry said in a statement it was "demanding an early release of the vessel." It did not immediately comment on Iran's allegation that the tanker had caused "oil pollution" in Gulf waters. 

Monday's seizure precedes an expected visit by South Korea's deputy foreign minister to Tehran.

Tensions over US sanctions

Iran has been demanding that South Korean banks release $7 billion (€5.7 billion) in funds frozen because of US sanctions re-imposed on Iran in 2018 after President Donald Trump withdrew from a 2015 multinational deal to restrain Iran's nuclear ambitions. 

The US Navy's Bahrain-based 5th fleet said authorities they were aware and monitoring the situation. 

Iran's semi-official Tasnim news agency said the tanker's crew members, including nationals from South Korea, Indonesia, Vietnam and Myanmar were being held in Bandar Abbas.

The ship seizure coincided with Iran's announcement that it had begun enriching uranium up to 20%at its Fordo underground facility — short of weapons-grade material. 

Israeli Prime Minister Benjamin Netanyahu warned that his nation would "not allow Iran to manufacture a nuclear weapon."

Tehran has long maintained that its nuclear program is peaceful.