TEHRAN (Reuters) - Iran's Revolutionary Guards said on Tuesday their naval forces had detained five Britons in the Gulf, the semi-official Fars News Agency reported.

British Foreign Secretary David Miliband said on Monday that five Britons had been detained in Iran and said their racing yacht might have inadvertently strayed into Iranian waters.

"The Britons have been arrested by the Revolutionary Guards' naval forces. 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.

Miliband said in a statement that the 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.

Relations between Britain and Iran 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.

In March 2007, Iranian forces seized eight British 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.

(Reporting by Ramin Mostafavi and Parisa Hafezi; writing by Fredrik Dahl; editing by Tim Pearce)