The 15 Royal Marines and sailors captured by Iran are being "interrogated", officials in Tehran said today.

Amid fears that the 14 men and one woman face a show trial, Iranian television reported that the country did not want to exchange the Britons for five Iranians arrested in northern Iraq two months ago.

The claim came as the Foreign Office said the prisoners were "fit and well" and said it was doing "everything possible" to secure their release. Iran's foreign ministry told British ambassador Geoffrey Adams that it was working to resolve the situation as soon as possible.

A Foreign Office spokesman described today's hour-long meeting as "business-like", with the ambassador pressing for details of where the detainees were being held.

Defence Secretary Des Browne told MPs today that his thoughts were with the crew members of HMS Cornwall being detained, their friends in theatre and the families in the UK.

He said: "We are doing everything possible to secure their release." But there were still fears that the captured patrol could face a show trial as Iran's president Mahmoud Ahmedinejad sought to exploit their arrest in his battle with the UN over the country's nuclear programme.

This morning's meeting with Mr Adams was the second in two days and follows Tehran's suggestion that the Britons could face prosecution for "illegally" entering Iranian waters.

The group was seized at gunpoint by Iranian forces on Friday in the Shatt al Arab waterway, which divides Iran and Iraq.

The eight sailors and seven Royal Marines from HMS Cornwall had just finished searching a vessel which they suspected of smuggling. The diplomatic row is over whether or not their two patrol craft strayed into Iranian waters.

The country's foreign minister Manouchehr Mottaki said that there were "legal issues" surrounding the alleged incursion over the country's borders - remarks interpreted as a signal that a trial could happen.

Iran's armed forces spokesman General Ali Reza Afshar said the crew had "confessed" to straying into Iranian waters.

Dr Ali Palaman, editor of the only free newspaper in Iran, told the BBC that he was convinced the threat of a show trial was being used as a bargaining chip.