(Adds possible U.N. vote, paragraph 4)
BASRA, Iraq, March 23 (Reuters) - Iranian forces seized 15 British servicemen on Friday in the mouth of the waterway that separates Iran and Iraq, triggering a diplomatic crisis at a time of heightened tensions over Tehran's nuclear ambitions.
Britain said two boatloads of sailors and marines had searched a cargo ship in Iraqi waters on a U.N. approved mission when Iranian gunboats encircled and captured them. No shots were fired and the British servicemen were unharmed, officials said.
Iran accused the British of illegally entering its waters.
The incident came as U.N. Security Council members were putting the final touches to a resolution imposing new sanctions on Iran over its refusal to halt sensitive nuclear work. A vote on the resolution could take place as early as Saturday.
Oil prices rose more than one percent to a three-month high on the news of the Britons' seizure.
Britain summoned Iran's ambassador in London and demanded the servicemen's immediate release.
"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," foreign minister Margaret Beckett said.
In Tehran, the foreign ministry summoned the British charge d'affaires.
"The Iranian Foreign Ministry has seriously objected following the illegal entry of British naval military forces into our country's waters," state television reported.
It said the Britons were detained by border guards for further investigation.
SIMILAR INCIDENT IN 2004
British officials were wary of drawing the conclusion that the incident was a deliberate provocation by Tehran, which denies Western accusations that its nuclear energy programme is a front for developing atomic arms.
"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.
It mirrored a similar event in 2004 when Iran seized eight British servicemen in the narrow waterway that separates Iran from Iraq and held them for three nights.
Then, as now, the Iranians accused the British of straying into Iranian waters, a charge Britain rejected.
"While there could have been some dispute over the whereabouts of the border that time, we are categorical that this time our people were operating clearly in Iraqi waters," a Ministry of Defence spokesman said.
He said those held by the Iranians comprised eight Royal Navy sailors and seven marines.
The incident 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.
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. London and Washington also say Iran foments violence in Iraq. (Additional reporting by Peter Graff, Sophie Walker, Paul Hughes and Randy Fabi in London, Edmund Blair in Tehran)
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