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TEHRAN/LONDON (Reuters) –
Iran will take serious measures against five British yachtsmen detained in the Gulf if it proves they had "evil intentions," a close aide to Iranian President Mahmoud Ahmadinejad said on Tuesday.

Relations between Britain and have been dogged by tension in recent years over a range of issues, from Tehran's nuclear program to Iranian allegations of British involvement in post-election violence in June this year.

Oil prices rose by over $1 on fears of a diplomatic crisis shortly after news of the detainment was made public on Monday.

"The judiciary will decide about the five...naturally our measures will be hard and serious if we find out they had evil intentions," Esfandiar Rahim-Mashaie, the president's chief of staff, told the semi-official Fars news agency.

Britain stressed the five men were civilians and played down parallels with a 2007 incident when Iran seized eight British Royal Navy sailors and seven marines off its coast.

"There is certainly no confrontation or argument. As far as we are aware these people are being well treated, which is right, and what we would expect from a country like Iran," British Foreign Secretary David Miliband BBC Radio 4.

Miliband said he was expecting a statement later on Tuesday from the Iranian Foreign Ministry.

"We understand that the Iranian government are investigating the incident, which is perfectly reasonable, and then we would look forward to it being promptly sorted out," Miliband said.

PROTESTS

Iran's Revolutionary Guards confirmed on Tuesday their naval forces had detained five Britons in the Gulf, Fars News Agency said.

"Confronting foreign forces and detaining them in the Gulf is the Revolutionary Guards' duty," said Ali Reza Tangsiri, a commander of the Guards' naval forces.

A new U.S. intelligence study says Iran has restructured its naval forces to give an arm of the elite Revolutionary Guards full responsibility for operations in the Gulf.

Miliband said the sailors may have "inadvertently strayed" into Iranian waters. Britain said their yacht was stopped by Iranian naval vessels on November 25.

Organizers of a race in which the yachtsmen were planning to take part said the vessel had reported problems with a propeller en route from Bahrain to Dubai in the Gulf.

Iran and Britain have a history of mutual suspicion.

Hard-line Iranian students will gather outside the British embassy in Tehran on Wednesday to protest "the Britons' illegal entry" into Iranian waters, the ISNA news agency reported.

In March 2007, Iranian forces seized eight Royal Navy sailors and seven marines in the mouth of the Shatt al-Arab waterway that separates Iran and Iraq. They were freed unharmed the following month.

Three Americans who crossed into Iran from Iraq in July are still detained and face spying charges. Their families say they were hiking and strayed across the border accidentally.

(Additional reporting by Ramin Mostafavi and Parisa Hafezi in Tehran; Writing by Jon Hemming; Editing by Angus MacSwan)