The ambush and capture of 15 British servicemen by Iranian Revolutionary Guards yesterday turned a tense situation in the Gulf into an explosive one.

In the past weeks, America and Britain have been increasing the pressure on the regime of Mahmoud Ahmadinejad, the Iranian President, in an attempt to curb his nuclear ambitions and to blunt Iran’s interference in neighbouring Iraq.

The Iranians have remained typically defiant throughout this campaign. Now they may be tempted to use the British sailors and Marines in their custody as bargaining chips against their Western detractors.

The stakes for all concerned could not be higher. The immediate risk is that the current military stand-off could escalate into something far more serious.

The US Navy has recently reinforced its task force in the region with a second aircraft carrier group deployed in the crowded waters of the Gulf, where America has assembled a flotilla of dozens of ships, hundreds of combat aircraft and thousands of sailors and marines.

The British have also contributed by sending naval reinforcements, including HMS Cornwall, the Type 22 frigate whose men were seized in yesterday’s incident.

Iran has responded to the show of force by launching more than a week of naval wargames in the Gulf, using small attack vessels and tactical submarines in a show of defiance against the West.

While the posturing would normally be dismissed as typical sabre-rattling by old foes, it comes at a time when relations have dropped to a dangerous new low.

President Bush and Tony Blair have both accused Iran of arming, training and supporting the insurgency in Iraq. The allegations were reinforced yesterday by Lieutenant-Colonel Justin Maciejewski, the commander of British forces in Basra city. He accused Iran of arming Shia militias with new weapons and of paying fighters £250 a month to attack British forces.

In December, US troops arrested and expelled two Iranians in Baghdad accused of being members of the al-Quds force, a unit responsible for foreign operations. Five other Iranians seized in January in northern Iraq on the same pretext remain in US custody.

Britain will be hoping that the release of its captured servicemen does not become entangled in the fate of the detained Iranians.

The tensions with Iran over Iraq and the Gulf are exacerbated by the diplomatic battle between the West and Tehran over the Iranian nuclear programme.

Under Mr Ahmadinejad, Iran has pushed ahead with mastering the enrichment of uranium, used as a fuel for a nuclear power station or, in its highly enriched form, the core of an atomic warhead.

America has supported attempts by Britain, France and Germany to seek a diplomatic solution, although it reserves the right to use force if a negotiated settlement fails and Tehran acquires a nuclear capability.

Last December, after months of diplomatic pressure, the United Nations Security Council passed a resolution imposing sanctions on Iran unless it halted its enrichment work.

The Iranians remained characteristically defiant. Now a second sanctions resolution is set to be passed today that bars Iran from exporting arms, restricts loans to Tehran and freezes the assets of 28 Iranian individuals and companies involved in nuclear or missile work.

Ayatollah Ali Khamenei, Iran’s Supreme Leader, gave warning this week that Tehran regarded the move as illegal and would respond by carrying out its own “illegal actions”.

British officials involved in a diplomatic offensive to win the quick release of the 15 missing servicemen are hoping that they are not the victims of Iran’s new policy.

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