The Iranian Foreign Minister accused a group of captured British servicemen last night of having committed an act of “aggression”, only hours after Tony Blair appealed for their release.

“The charge against them is their illegal entrance into Iranian territorial waters,” Manouchehr Mottaki, the Foreign Minister, told a press conference in New York.

In a telephone conversation with Mr Mottaki last night Margaret Beckett, the Foreign Secretary, “made extremely clear our view that our personnel were operating in Iraqi waters, called for their immediate return, and asked for immediate consular access to them”, a spokesman said.

But Mr Mottaki told the conference that Iran had already provided British officials with details, including GPS coordinates, of the servicemen’s arrest. The British Ambassador to Tehran was summoned to the Foreign Ministry to explain why 15 service personnel in two inflatable boats had strayed into Iranian territorial waters.

“The Iranian authorities intercepted these sailors and Marines in Iranian waters and detained them in Iranian waters. This has happened in the past as well. In terms of legal issues, it’s under investigation,” Mr Mottaki said. His comments were seen as a direct rebuff to the Prime Minister, who only hours earlier had called the seizure of the British servicemen “unjustified and wrong” and demanded their release.

“This is a very serious situation and there is no doubt at all that these people were taken from a boat in Iraqi waters,” Mr Blair said. “It is simply not true that they went into Iranian territorial waters and I hope the Iranian Government understands how fundamental an issue this is for us. We have certainly sent these messages back to them very clearly indeed. They should not be in any doubt over how seriously we take this act, which was unjustified and wrong.”

Mr Mottaki flew to New York for Saturday’s unanimous UN vote to tighten sanctions on Tehran over its suspected nuclear weapons programme, after President Ahmadinejad abruptly cancelled his trip. Sir Emyr Jones-Parry, Britain’s UN Ambassador, tried unsuccessfully to raise the subject of the servicemen’s fate with the Iranian Foreign Minister.

Behind the scenes, British diplomats worked furiously over the weekend to increase the pressure on Iran by appealing to the regime through friendly third parties.

Geoffrey Adams, the British Ambassador to Tehran, asked the Iranian Foreign Ministry yesterday where the 15 captives were being held and demanded consular access to them.

So far the Iranians have refused to give any details about their fate, other than to say that they are being well treated. General Ali Reza Afshar, Iran’s armed forces spokesman, said that they had been taken to Tehran for questioning and that they had “confessed” to an “aggression into the Islamic Republic of Iran’s waters”.

Diplomats involved in the case believe that the British servicemen were ambushed by a naval unit of Iran’s Revolutionary Guards with the intention of putting pressure on Britain ahead of the key UN Security Council vote to impose sanctions on Tehran for its nuclear programme. If that was the motive, it failed. On Saturday, the day after the abduction, the council voted unanimously to impose sanctions on Iran, banning the export of weapons and freezing the assets of 28 individuals and companies involved in the country’s nuclear and ballistic missile programmes.

The resolution has given Iran 60 days to freeze its uranium enrichment work or it could face further sanctions.

Iran said last night that it would limit cooperation with the UN’s nuclear watchdog and vowed not to halt its atomic programme “even for one second”.

John Bolton, who until recently was the US envoy to the UN, said he believed that the seizure of the British sailors was a “conscious decision by the Tehran Government”, related to the Security Council’s planned sanctions against Iran.

He added: “They were possibly picking on the British because they think the Europeans are the weak link in this.”

2004 capture

— June 18, 2004 Tehran furious at critical International Atomic Energy Agency resolution

— June 21 Eight Royal Marines captured in Shatt al-Arab waterway

— June 22 Iranian TV shows the British boats. The eight are paraded on TV blindfolded and Sergeant Thomas Harkins reads out apology

— June 23 Jack Straw, Foreign Secretary, demands the soldiers’ release

— June 24 Iran frees soldiers but keeps captured boats

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