South Korea and the United States have demanded the release of a South Korean-flagged oil tanker seized by Iranian authorities in the Strait of Hormuz.
The MT Hankuk Chemi and its crew of 20 were stopped on January 4 over alleged "oil pollution" in the Persian Gulf and in the strait.
The semiofficial Fars news agency said Iran's Islamic Revolutionary Guards Corps (IRGC) naval forces seized the ship.
The IRGC said on their Sepah News website that the ship carrying 7,200 tons of oil chemical products "was seized by our force's [navy] this morning."
"This tanker was headed from Saudi Arabia's Al-Jubail port and was seized due to the repeated infringement of maritime environmental laws," it added, quoting an IRGC statement. "Its case will be handed over to the judicial authorities of the country."
Satellite data from MarineTraffic.com showed the MT Hankuk Chemi off the port of Bandar Abbas. It had been traveling from Saudi Arabia to Fujairah in the United Arab Emirates.
South Korea's deputy foreign minister is due to visit Tehran in the coming days, according to Iranian officials, who want South Korea to release billions of dollars in assets held because of U.S. sanctions.
The U.S. State Department accused Tehran of threatening freedom of navigation in the gulf as a way to extract relief from economic sanctions.
The Iranian regime "continues to threaten navigational rights and freedoms in the Persian Gulf as part of a clear attempt to extort the international community into relieving the pressure of sanctions," said a State Department spokesperson on condition that they not be identified, Reuters reported. "We join the Republic of Korea's call for Iran to immediately release the tanker."
The incident comes with tensions on the rise in the region coinciding with the anniversary of the U.S. drone strike that killed Major General Qasem Soleimani, the commander of the elite Quds Force of the IRGC, in Baghdad last year.
That attack later saw Iran retaliate by launching a ballistic-missile strike that injured dozens of U.S. troops in Iraq. Tehran also accidentally shot down a Ukrainian passenger jet that night, killing all 176 people on board.
As the anniversary approached, the United States sent B-52 bombers flying over the region as well as a nuclear-powered submarine into the Persian Gulf.
The Iranian government also said on January 4 that the country had resumed 20 percent uranium enrichment at an underground facility, a level far above limits set by the 2015 nuclear deal with six major powers.
Government spokesman Ali Rabiei said President Hassan Rohani gave the order for the move at the Fordow facility -- the latest of several recent Iranian breaches of the international agreement that eased UN sanctions in exchange for curbs on Iran's disputed nuclear program.
Tehran has gradually reduced its compliance with the accord since the United States unilaterally withdrew from the deal in 2018 and started imposing crippling sanctions on Iran.
Enriched uranium can be used to make reactor fuel but also nuclear warheads, with 90 percent purity considered weapons-grade.
Tehran has always denied pursuing nuclear weapons, saying its nuclear program was strictly for civilian purposes.