and CAROLYN CHURCHILL

IRAN'S chief international negotiator signalled last night that the row over the 15 captured British sailors and marines could be resolved through diplomacy.

Ali Larijani said there was no need to put the crew on trial and Iran's priority was "to solve the problem through proper diplomatic channels". His comments came as officials in the US said they were asking Iran for information about a former FBI agent who is believed to have gone missing several weeks ago.

The agent, who was on private business in Iran, was reported missing by his family and employer.

His disappearance came after tentative signs of a breakthrough emerged in the diplomatic crisis between Britain and Iran.

Earlier, Iran said it had noted positive changes in Britain's attitude and promised not to show fresh pictures of the 15 sailors.

Senior Whitehall sources told The Herald that, following an exchange of messages, the government was waiting for a statement by Mahmoud Ahmadinejad, the Iranian President, today at the end of the country's prolonged New Year public holiday.

Tony Blair's spokesman said: "There is a lot going on behind the scenes but it is better I don't go into that in any way. At this stage, it is best to wait and see what the Iranian response is."

He did not contradict the suggestion that the 15 sailors were hostages and was emphatic that Britain was not engaged in negotiations. He said: "Iran knows our position. They know stage-managed TV appearances are not going to affect our position. They know we have strong international support."

When it was suggested the government, 11 days into the crisis, still did not know where the sailors were held, the spokesman replied: "I'm not getting into that," adding: "What happens next depends on the Iranian response."

Earlier, Iranian state television showed more pictures of the captured sailors, standing in front of maps, supposedly confessing their transgression into Iranian waters - an assertion flatly denied by the British government. The footage cut quickly between several of the men, who were not heard to say anything.

State-run radio reported that further personal confessions by the servicemen were not being broadcast because of "positive changes" in Britain's stance. However, Iran claimed all 15 UK service personnel had confessed to illegally entering the country's waters. But last night Iran seemed to have moderated its tone. Mr Larijani, the secretary of Iran's Supreme National Security Council, who had indicated last week that the case may "face a legal path", was more conciliatory.

He said that Iran wanted a quick conclusion to the crisis. "We are not interested in letting this issue get further complicated," he said. "We definitely believe that this issue can be resolved and there is no need for any trial."

Mr Larijani, the top negotiator in all of Iran's foreign dealings, said in Tehran that there should be a delegation to clarify if the 15 had strayed into Iranian waters. He also called for all those involved to stop using "the language of force".

He said: "There is a difference of view between the UK government and the Iranian government and this issue should be resolved bilaterally.

"Definitely our priority would not be trial," he added, unless "the UK government would be insisting on not solving the problem through diplomatic channels".

For Britain's part, "a guarantee must be given that such a violation will not be repeated", he added.

Larijani also called for a delegation "to review the case, to clarify the case, first of all - to clarify whether they have been in our territorial waters at all". He did not say who might be in such a delegation.

He added: "Through sensationalism, you cannot solve the problem."

The 15 British personnel were captured by Iranian forces in the Persian Gulf on March 23. Iran says that they were in its territorial waters but Britain insists they were in Iraqi territory.

Meanwhile, the US State Department said it was sending an official inquiry to Iran over the missing former FBI agent via Swiss diplomats, who act as a go-between with Tehran because the two countries do not have diplomatic relations.

FBI spokesman Rich Kolko said the agent retired nearly a decade ago. He said the missing man was last seen in Iran early last month and was not working for the FBI.

"There are no indications that this matter should be viewed other than as a missing person case," Mr Kolko said.

He also said the former agent had worked on traditional criminal issues, such as organised crime cases, not international terrorism or intelligence work that could have taken him to Iran.