Britain confirmed it had "lost contact" with three vessels and eight sailors in the narrow Shatt al-Arab waterway that separates southwestern Iran from Iraq. Britain had small naval ships there helping to train Iraqi police.
"We can confirm we did lose contact with a patrol earlier this morning and it does involve three small vessels and eight personnel," a Defense Ministry spokesman said. He could not confirm that Iran had seized them.
Britain stressed that the boats in question are not large warships.
"We are not talking about warships, we are talking about small river patrol vessels," the spokesman said.
The British Broadcasting Corp. reported that a spokesman for the U.K. Foreign Ministry said diplomats from the two countries have been discussing the incident, but that Tehran had not yet confirmed the identities of the crewmembers.
This incident will probably further strain Tehran's ties with London, which last week joined other European nations in condemning Iran for not fully cooperating with inspectors from the U.N. nuclear watchdog agency.
Iran Foreign Ministry spokesman Hamid Reza Asefi said in a statement Monday, "This morning three British vessels with eight crew entered the Islamic Republic of Iran's waters and Iran's naval forces, acting on their legal duty, confiscated the vessels and arrested the crew.
"The crew are under investigation in order to clarify the issue," he added.
Tehran did not indicate when or whether the British ships and crew might be released.
Iran's state television reported maps and weapons carried on the British vessels were confiscated.
Massoud Jazaeri, a spokesman for Iran's Revolutionary Guards, told Reuters Iran was determined to defend its territorial integrity.
"Anyone from any nationality entering our waters will face the same response," Jazaeri said.