Tony Blair warned Iran yesterday that the dispute over the 15 British servicemen seized in Gulf waters last week could move into a “different phase” if diplomacy failed to secure their release.

His words, immediately condemned by Iran as “provocative”, came as the US Navy began its biggest show of force in the Gulf since the invasion of Iraq four years ago, with manoeuvres involving two aircraft carriers, a dozen warships and more than 100 aircraft.

As tensions rose, Margaret Beckett, the Foreign Secretary, had a robust telephone conversation with her Iranian counterpart demanding immediate consular access to the captured Britons.

In an interview on GMTV, Mr Blair said: “I hope we manage to get them to realise they have to release them. If not, then this will move into a different phase.”

Asked what he meant, he sqaid: “Well, we will just have to see but what they should understand is that we cannot have a situation where our servicemen and women are seized when actually they are in Iraqi waters under a UN mandate, patrolling perfectly rightly and in accordance with that mandate, and then effectively captured and taken to Iran.”

His spokesman later said that he had been referring to a “different” way of handling talks, such as showing why Britain knew that the two boats were outside Iran’s waters.

The decision to offer no resistance to Iran, although the Britons were said to have been operating in Iraqi waters, was down to the commander of the two boats. There was no air cover at the time because a helicopter had just returned to HMS Cornwall after watching the successful boarding of a merchant vessel.

Britain has Iraqi backing in its insistence that the 15 sailors and Marines were on “routine” anti-smuggling operations in Iraqi waters when they were seized at gunpoint in the Shatt al-Arab waterway. Iran says that they entered its territorial waters illegally.

Iran gave a warning that Mr Blair’s comments would only aggravate tensions. “The media campaigns and provocative . . . remarks regarding the violation of Iranian territorial waters by the British sailors are doing nothing to help settle the affair,” Mohammad Ali Hosseini, a Foreign Ministry Spokesman, said. “The British service personnel entered Iranian waters illegally and the case will follow its legal and judicial course.”

The American exercises included rapid-fire simulated air attacks. US Navy Commander Kevin Aandahl said that the operation would last several days and was not meant to be seen as a response to the capture of the British sailors — nor to threaten Iran. He added that warships would be staying out of Iranian territorial waters.

Diplomatic sources in Washington yesterday suggested that such sabre-rattling was designed to reassure other Gulf states such as the Sunni-led Saudi Arabia and the UAE, which are increasingly alarmed at the prospect of Shia Iran asserting itself across the region.

Pentagon military strategists have long since identified the threat Iran poses with its considerable naval strength to close the narrow Straits of Hormuz in the Gulf, through which much of the world’s oil supplies flow. The Islamic Republic conducted its own naval manoeuvres twice last year, in April and November.

The US military exercises this week involve more than 10,000 personnel making simulated attacks on enemy aircraft and shipping, hunting submarines and finding mines.

Commander Aandahl said that it was for “regional stability and security”. He added: “If there’s a destabilising effect, it’s Iran’s behaviour.”

Admiral William Fallon, the top US commander for the Middle East, used an interview with CNN to restate American claims that Iran is backing Shia militias in Iraq.

“The question is, what is Iran going to do about its behaviour?” he said. “Iranian behaviour has been not only unhelpful, but detrimental to peace and progress.”

US commanders have suggested that the British sailors should have fought rather than allow themselves to be captured. Lieutenant-Commander Erik Horner, second-in-command on USS Underwood, the frigate working in the British-controlled task force with HMS Cornwall, said: “We not only have a right to self-defence but also an obligation to self-defence. [The British] had every right and every justification to defend themselves rather than allow themselves to be taken. Our reaction was, ‘Why didn’t your guys defend themselves?’ ”

Mr Hosseini said that British diplomats would be able to meet the 15 once investigators had completed questioning them about what they had been doing in Iranian waters.

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