Iran said on Tuesday that five British sailors detained in southern Gulf waters by the elite Revolutionary Guards would be dealt with firmly if found guilty of what it called "ill intentions."

The five men on board an 18-metre (60-foot) racing yacht were detained last week by the Revolutionary Guards, the force's navy chief told the Fars news agency.

It quoted President Mahmoud Ahmadinejad's chief of staff, Esfandiar Rahim Mashaie, as saying the judiciary would decide their fate.

"If these people's ill-intention is proven, they will be dealt with seriously and firmly," he said, adding that Tehran had in the past dealt firmly with people who "entered Iran illegally."

Related article: Britons held in Iran 'tough', say families.

In response, British Foreign Secretary David Miliband said "there's certainly no question of any evil intent," insisting that the five were clearly innocent and saying he hoped for a speedy resolution.

He also said the incident has "nothing to do" with politics or the standoff over the Islamic republic's nuclear programme, which the West suspects has military aims despite Tehran's denial.

"This is a human story of five young yachtsmen," Miliband told reporters in London. "It's a consular case, which is being treated as a consular case by the UK, and I'm sure will be treated as a consular case by the Iranian authorities."

In London the five were named as Oliver Smith, Oliver Young, Sam Usher, Luke Porter and sports journalist David Bloomer, who holds dual Irish-British nationality.

London summoned Iranian envoy Rasoul Movahedian over the detention.

A Foreign Office spokesman said Britain had sought confirmation of where the five were being held and asked for consular access and a "swift resolution."

In a statement afterwards, the department said: "Since it was now almost a week since the yachtsmen were held, the delay in providing a full explanation of what had happened and clarity on Iranian intentions were a matter of increasing concern."

Separately, a Foreign Office spokeswoman told AFP Britain believed the group was being held on Sirri Island in the Gulf off the Iran coast.

"We understand that they are being held on Sirri Island but we are seeking confirmation of that with the Iranians," she said.

Heightened tensions

The seizure, which recalls the detention by Iran of 15 British navy personnel in the Gulf in 2007, comes amid already heightened tensions between Tehran and the West over Iran's nuclear plans.

The five were sailing from Bahrain to Dubai for a race when their yacht, "The Kingdom of Bahrain," was stopped last Wednesday in the Gulf, the Foreign Office said.

They are believed to have been intercepted near the Iranian-controlled island of Abu Musa, whose ownership is disputed by Iran and the United Arab Emirates, a Bahraini interior ministry source told AFP.

Bahrain also said it was "contacting" the Iranian authorities over the release of the Britons.

Miliband stressed that the latest incident was different from the one in 2007, which involved military personnel.

"It is important to say that these are civilians, not Royal Naval personnel," he said. "They are yachtsmen, they were going about their sport and it seems they may have strayed inadvertently into Iranian waters."

Andrew Pindar, chairman of the Sail Bahrain team which owns the yacht, said in a statement it may have drifted into Iranian waters because of a damaged propeller.

"We received a call from the crew stating that they had been stopped by an Iranian navy vessel," he said.

"We understood that the crew believed they were in UAE waters, but due to a fault with the propeller, they may have inadvertently drifted into Iranian waters."

Revolutionary Guards navy chief Ali Reza Tangsiri told Fars that the five "intruders" had been arrested by his forces.

Charles Porter said he had spoken to his 21-year-old son Luke on a mobile phone and that he appeared to be in good spirits.

"From what we understand there was an oilfield on their charts -- which is a restricted area -- so they chose to go one side of it," Porter said, adding the yacht may have strayed too close to an Iranian island.

In the 2007 incident, eight sailors and seven marines were captured on March 23. Britain insisted they were in Iraqi territorial waters, while Tehran said they were in Iranian waters.

During the 13 days they were held, the 14 men and one woman were not mistreated but they were paraded on Iranian television, sparking anger from Britain and other Western governments.