"Overnight, US 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.

The Tifani is a "Botswana-flagged tanker," according to the intelligence firm Vanguard Tech, which said the vessel was intercepted in the Indian Ocean.

Its last signal was detected on Tuesday halfway between Sri Lanka and the Strait of Malacca, according to the maritime tracking website Marine Traffic.

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

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

According to energy intelligence firm Kpler, the vessel had loaded approximately two million barrels of crude on Iran's Kharg Island on April 5 and passed through the Strait of Hormuz on April 9. Its tracking signal indicated it was heading toward Singapore.

The Tifani has in recent years carried out numerous ship-to-ship oil transfers off Singapore and Malaysia and made multiple round trips between this area and destinations including Iran and China.

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

On Monday, however, maritime data firm Lloyd's List Intelligence said "at least 26 ships from Iran's ghost fleet [had] circumvented the US blockade" since it was imposed last week.

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