Iran's most senior diplomat, Ali Larijani, called for a "delegation" to rule on whether a British naval patrol entered Iranian waters last month before his government would release the 15 marines and sailors it is holding captive.

Laying out what appeared to be a vague road map for the freeing of the British personnel, Mr Larijani said that, if it was found they had crossed into Iranian territory, there should be an apology and they would then be released.

He gave some conciliatory signals in an interview with Channel Four News, saying the Iranian government was not interested in putting the detainees on trial, but warned that might change if Britain attempted to impose more international pressure on Tehran. "We are not interested in this issue getting more complicated," said Mr Larijani, the secretary-general of Iran's national security council.

"Our interest is in solving this problem as soon as possible. This issue can be resolved, and there is no need for any trial. There should be a delegation to review the case ... to clarify whether they have been in our territorial waters or not."

Mr Larijani did not specify whether the delegation he was requesting should be British or international, but he did say the issue should be solved "bilaterally". His remarks could be a response to an offer by Britain to send a team of naval experts and diplomats to discuss how to avoid a repetition of the crisis. A Foreign Office spokeswoman said last night: "We are still studying Dr Larijani's remarks.

"There remain some differences between us, but we can confirm we share his preference for early bilateral discussions to find a diplomatic solution to this problem. We will be following this up with the Iranian authorities tomorrow, given our shared desire to make early progress."

However, British officials are adamant that the team of experts would not be going to negotiate the captives' release, and would focus on the future rather than on the March 23 incident. They said proposed talks would ideally improve the current atmosphere, but would not include acceptance of Iranian claims that the British patrol had entered Iranian waters.

Earlier in the day, Iranian media noted "positive changes" in negotiations with Britain over the crisis. They said that was the reason they did not broadcast "confessions" of a territorial incursion by all 15 captives, which Iran says it has recorded. So far, four have been shown "admitting" that they had entered Iranian waters.

The head of Iran's parliamentary committee on foreign policy and national security, Allaeddin Broujerdi, seemed to echo the British suggestion for talks yesterday when he told state radio: "There is a need for a bilateral agreement to prevent such an event in the future."

In seeking the captives' release, Britain has been seeking help from Iran's allies. Syria's president, Bashar al-Assad, has intervened, the Guardian has learned. Mr Assad raised the issue with the Iranian foreign minister, Manuchehr Mottaki, at the Arab summit conference in Riyadh last Wednesday. It came shortly before Mr Mottaki told an Iranian TV station that the captured sailor Leading Seaman Faye Turney would be released shortly. The move followed a direct appeal to Damascus by Sir Nigel Sheinwald, Tony Blair's chief foreign policy adviser.

It came only five months after Sir Nigel visited the Syrian capital in an attempt to persuade Mr Assad to distance himself from Iran. British officials have been impressed by Syria's readiness to help in the dispute with Iran, and have singled it out for praise in recent days.

John Bolton, the Bush administration's former ambassador to the UN, yesterday criticised the British government for its "weak" and "passive" response to Iran over the captives. "If I were sitting in Tehran, I would say, 'I played this card against the Brits and they did everything but plead with me to give these people back'," he told CNN. "I think that tells the Iranians quite a bit about European resolve."