Fifteen British Royal Marines on patrol in the Persian Gulf have been “seized” by the Iranian navy, the British Ministry of Defense said.

The Marines were “engaged in routine boarding operations of merchant shipping in Iraqi territorial waters,” the ministry said in a statement on Friday.

A U.S. military official who monitors the region told CNN the Marines stopped an Iranian ship suspected of smuggling automobiles, and boarded it for an inspection.

While the Marines were on board, six Navy ships from the Iranian Revolutionary Guard showed up and claimed the British had entered Iranian waters.

A dispute ensued over whether the Marines were in Iraqi, international, or Iranian territorial waters, and the 15 were then seized and taken to Iran, the U.S. military official said.