The families of five British sailors being held captive in Iran spoke of their concern today after reports suggested the crew faced "serious" action if found to have had "evil intentions".
The Kingdom Of Bahrain racing yacht, owned by Sail Bahrain, was stopped by the Iranian navy last Wednesday as it sailed from Bahrain to Dubai.
Bahrain-based radio presenter David Bloomer, Oliver Smith, 31, from Southampton, Oliver Young, 21, from Plymouth, Sam Usher, and Luke Porter, 21, were on board.
Esfandiar Rahim-Mashaie, Iranian president Mahmoud Ahmadinejad's head of staff, told the country's Fars news agency: "Judiciary will decide about the five ... naturally our measures will be hard and serious if we find out they had evil intentions."
The Foreign Office said it was looking into the reports.
Foreign Secretary David Miliband said there was "no confrontation or argument" in negotiations with Iran.
But Mr Porter's father Charles, of Weston-super-Mare, said he and his wife Beverley were worried.
"We are holding things together as a family at the moment," he said.
"I haven't spoken to him since yesterday. He was as good as can be expected.
"He is a very strong character, very resilient. He's a professional sailor, very used to dealing with adversity.
"He's as good as we can expect. We are very concerned."
He said his son had been in Bahrain for a couple of months.
Mr Young's father, Plymouth businessman David Young, told the Plymouth Herald he found out on Thursday night that his son had been detained.
He said: "He's travelled a fair bit. He's got quite a bit of experience under his belt. They'll be coping with it fine out there. We understand they are being well looked after.
"We're confident this will be resolved very quickly. We're all concerned and the sooner they are released, the better. It is dragging along longer than we thought it would do.
"It's just a worry that there are diplomatic stresses at the moment. They are under international pressure. We just hope they're not used as a bargaining chip."
The capture of the sailors emerged after Iran unveiled plans to build 10 new uranium enrichment plants in breach of United Nations resolutions, and follows a period of prolonged tension between Iran and the West.
But Mr Miliband, speaking on BBC Radio 4's Today programme, stressed the incident was simply a consular matter.
"These are five civilians. They are yachtsmen. They were going about their sport," he said.
"It seems they may have strayed inadvertently into Iranian waters. We look forward to the Iranian government dealing with this promptly."
He said he was awaiting a statement from the Iranian government on the matter but stressed there was no dispute.
"There's certainly no confrontation or argument," he said. "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."
He said "perfectly good discussions" had taken place between officials in London and Tehran.
Sir Richard Dalton, the former UK ambassador to Iran, told BBC News: "I think there would be much less reason for the Iranians to think that making political capital out of the incident is of benefit to them."
But Ben Wallace, chairman of the British-Iranian All-Party Parliamentary Group, said the actions of the Iranian authorities may be a deliberate move to gain political leverage.
The Tory MP for Lancaster and Wyre told the BBC: "This is more than just saying 'Get out of our backyard', this is odd behaviour, but is in line with Iranian behaviour. When (Iran) wants to make a point, it takes hostages."
The sailors were heading to Dubai to join the Dubai-Muscat Offshore Sailing Race.
Sail Bahrain was recently launched by yachting company Team Pindar.
The boat had been due to take part in the 360-mile race, which was due to start last Thursday.
A Team Pindar statement read: "On November 25, Sail Bahrain's Kingdom of Bahrain Volvo 60 racing yacht was stopped by Iranian navy vessels as it was making its way from Bahrain to the start of the Dubai-Muscat Offshore Sailing Race.
"The boat may have strayed inadvertently into Iranian waters.
"The five crew members, all British nationals, are still in Iran.
"All are understood to be safe and well and their families have been informed."
Keith Mutch, general manager of the Dubai Offshore Sailing Club, told the Today programme: "The yacht was on its way from Bahrain to Dubai. En route there is a number of Iranian oilfields and it could well have strayed into one of those oilfield waters."
Mr Smith is an experienced sailor. He completed a degree in ocean science and marine navigation at the University of Plymouth while sailing with the university men's first team.
He then spent six months skippering a 50ft survey yacht and doing delivery work in South America and the Caribbean.
He was part of a team which came third in the Racing Division of the ARC (Atlantic Rally for Cruisers), and in 2003 went on to work for Pure Sailing as the boat captain and skipper for a Volvo 60 raceboat.
Mr Bloomer, who is believed to hold dual British and Irish nationality and is said to be in his 60s, was due to broadcast updates on the yacht's progress in the race.
On November 19, Mr Usher posted a message on Facebook which read: "Hi Lads, see you in a few days for the event. hope all has been going ok!"
Ireland's Department of Foreign Affairs said Mr Bloomer was travelling on a British passport, although he is believed to be from Malahide, Co Dublin.
It is believed the radio presenter and sports journalist has been living in Bahrain since the 1980s.
Mr Young's mother, Susan, said she was confident that her son would cope well with the pressure of the situation.
Mrs Young, of Saltash, Cornwall, said: "He will be absolutely fine. He is a very strong person, he will be coping with this fine.
"The team get on really well, they are a really nice group of lads and I am not worried about their morale or anything like that.
"As a mother I do feel worried but who wouldn't be? We are just hopeful that it will all come to an end very soon."
She said Oliver was one of four brothers and had always been interested in sailing.
She said: "It's a complete passion for him and this will never put him off, he will be fine."
Nicola Drayton, fiancee of Sam Usher, said it was a difficult time for her and their two children.
Miss Drayton, of Scarborough, North Yorkshire, said she was in contact with the Foreign Office about her partner's situation in Iran.
She said: "It's difficult but you just get on, you have no choice."
She added that Mr Usher was the owner of Wykeham Watersports, a sailing academy on the Dawnay Estate, north east of York.