"Overnight, U.S. forces conducted a right-of-visit, maritime interdiction and boarding of the stateless sanctioned M/T Tifani without incident in the INDOPACOM area of responsibility," the department posted on its X page.

The statement said the United States was determined "to disrupt illicit networks and interdict sanctioned vessels providing material support to Iran -- anywhere they operate."

The exact location of the operation was not made clear. INDOPACOM is the US Indo-Pacific Command, which oversees a broad region that includes the Pacific and Indian oceans.

It is not responsible for the Gulf or the Strait of Hormuz, which fall under US Central Command.

"International waters are not a refuge for sanctioned vessels," the Pentagon said in its post, which included video footage showing helicopters landing on a large, bright orange tanker.

AFP has identified the vessel as one affiliated with Iranian activity.

US President Donald Trump has vowed to maintain a blockade on Iran "until there is a deal" to end the war.

Doubts remained Tuesday regarding whether a second round of negotiations between Tehran and Washington would take place in Islamabad.