Iran's ambassador Rasoul Movahedian leaves the Foreign Office in London yesterday. Movahedian was summoned to the Foreign Office yesterday over the seizure of 15 British navy personnel by Iranian forces in the Gulf. - Reuters

BASRA, Iraq (Reuters):

Iranian forces seized 15 British servicemen yesterday in the mouth of the waterway that separates Iran and Iraq, triggering a diplomatic crisis at a time o tensions over Tehran's nuclear ambitions.

Britain said two boatloads of sailors and marines from the naval frigate Cornwall had searched a merchant vessel in Iraqi waters on a United Nations-approved mission when Iranian gunboats encircled and captured them.

Extremely disturbed

British Foreign Secretary Margaret Beckett said she was "extremely disturbed". Britain summoned Iran's ambassador in London and protested through its embassy in Tehran.

"We sought a full explanation of what happened and left the Iranian authorities in no doubt that we expect immediate and safe return of our service personnel and boats," Beckett said.

Washington backed its ally. "We support the British demand for the safe return of their people and equipment," State Department spokesman Sean McCormack said.

Tehran maintained silence throughout the day. The incident sent oil prices up more than one per cent to a three-month high.

Naval war games

It took place a day after Iran launched a week of naval war games along its coast, including the Gulf's northern reaches which give access to the oil output of Iraq, Iran and Kuwait.

"There was no fighting, no engagement of weapons, anything like that, it was entirely peaceful," said Commodore Nick Lambert, commander of the British fleet in the area.

"We have been assured from the scant communication we have had with the Iranians at a tactical level that the 15 people are safely in their hands," he said aboard the Cornwall.

British officials were wary of drawing the conclusion that the incident was a deliberate provocation by Tehran.

"This may well be a misunderstanding. We're certainly treating it as such at the moment. We're looking for the mistake to be corrected," a British government source said.

Unlike the United States, Britain has diplomatic relations with Iran. But London backs Washington's calls for tough sanctions against Iran over its nuclear programme.