(Reuters) - Iran's Revolutionary Guards "forcibly seized" a commercial ship in international waters in the Gulf on Thursday and the vessel was possibly involved in smuggling, a U.S. Navy spokesperson said.

The U.S. Navy had monitored the situation and decided not to make any further response, U.S. 5th Fleet spokesperson Commander Tim Hawkins said.

British maritime security company Ambrey said it was aware of an attempted seizure by Iranian forces of a small Tanzanian flagged tanker, around 59 nautical miles northeast of the Saudi Arabian port city of Dammam.

"Iran regularly intercepts smaller tankers it suspects of smuggling oil," the company added in a note.

About a fifth of the world's supply of seaborne crude oil and oil products passes through the Strait of Hormuz, a chokepoint between Iran and Oman, according to data from analytics firm Vortexa.

The U.S. Navy said on Wednesday that it had intervened to prevent Iran from seizing two commercial tankers in the Gulf of Oman, in the latest in a series of attacks on ships in the area since 2019. 

"U.S. forces remain vigilant and ready to protect navigational rights of lawful maritime traffic in the Middle East’s critical waters," Hawkins said.

Iran said on Thursday it had a court order to seize one of the tankers sailing in Gulf waters on Wednesday after it collided with an Iranian vessel. The vessel, the Bahamas-flagged Richmond Voyager, was managed by U.S. oil major Chevron (CVX.N).

Tehran seized two other tankers in May including the Marshall Islands flagged Advantage Sweet, which had been chartered by Chevron.

Since 2021, "Iran has harassed, attacked or seized nearly 20 internationally flagged merchant vessels", the U.S. Navy said this week.

Reporting by Jonathan Saul; Editing by Hugh Lawson and Andrew Heavens