An Iranian naval patrol seized 15 British sailors who had boarded a vessel suspected of smuggling cars off the coast of Iraq, military officials said.
The British government immediately demanded the safe return of its troops and summoned Tehran's London ambassador to explain the incident.
The Royal Marines and ordinary naval officers were believed to have been apprehended by up to six ships from the Iranian Revolutionary Guard Corps Navy who claimed they had violated Iranian waters.
British military patrols have been given authority to board vessels in Iraqi waters under United Nations mandate and with the permission of the government in Baghdad.
He said the captain of the merchant vessel had been cleared to proceed and the two British inflatable patrol boats were readying for departure when they were surrounded by the Iranian navy and taken into Iranian waters.
Lambert said there is "absolutely no doubt in my mind" that the marines were in Iraqi waters. But, he said, "The extent and the definition of territorial waters in this part of the world is very complicated... We may well find, and I hope we find, that this is a simple misunderstanding at a tactical level," he said.
"There hopefully has been a mistake that's been made, and we'll see early clarification and early release of my people."
CNN's Aneesh Raman in Tehran said there had been no mention of the incident on Iranian TV and calls to officials had not been answered.
It was not immediately clear where in Iran the British personnel were taken.
This is exactly the type of thing that could spin out of control quickly..
Questions: Is this an intentional act approved by senior Iranian leadership in response to findings of the British personnel, or possibly in reaction to the upcoming U.N. vote against Iran? The official IRNA news site includes a story complaining that the White House is throwing up a last-minute obstacle to the issuance of a visa for President Ahmadinejad to take part in the U.N. Security Council meeting Saturday on the Iran sanctions resolution - could that be the reason for this action? Is this a provocation similar to the Hezbollah seizure last year of Israeli soldiers, which led the Israelis into invading Lebanon, to test how the British and Americans move military assets in advance of armed action? Is this a calculated measure due to Iranian claims that the waters are, in fact, Iranian and not Iraqi (a 1975 treaty gave the waters to Iraq, but Iran disputes Iraq's jurisdiction)? Or is this the action of a local commander, unauthorized by leadership, and due to anything from bad navigation equipment (hard to believe but it happens), one too many drinks, or a misinterpretation of orders? Recall that (a) Iranian forces did something like this in 2004 and held British servicemen for three days, then released them, and (b) local commanders' mistakes have had devastating consequences, such as the accidental American shoot-down of an Iranian civilian airliner in 1988.
