When US forces burst into a villa and arrested five Iranian men in the northern Iraqi city of Arbil this year, they may have set in motion a chain of events that led directly to the abduction of 15 British servicemen in the northern Gulf last week.

While the British and Iranian governments argue about whether the sailors and Marines were in Iraqi or Iranian waters at the time of their capture, privately there is acknowledgement that their fate is bound closely to that of the Iranian captives.

As part of a campaign to crack down on Iranian influence in Iraq, President Bush ordered US forces to root out Iranian agents suspected of arming and funding Iraqi Shia militias. The Iranian liaison office in the Iraqi Kurdish region was an obvious choice. When US troops stormed the building they found Iranians trying to flush fake documents down the lavatory. All five were allegedly members of the al-Quds unit of the Iranian Revolutionary Guards Corps. The unit is responsible for promoting the Iranian revolution abroad by assisting militant groups with funding, training and arms.

Iranian officials speculated that the way to win the freedom of their comrades was to capture American or British soldiers and arrange a prisoner swap. Reza Faker, a writer for the Revolutionary Guards’ newspaper Subhi Sadek, said: “We have the ability to capture a nice bunch of blue-eyed blond-haired officers and feed them to our fighting cocks.” Reza Zakeri, of President Ahmadinejad’s office, said that capturing a Western soldier was easier than acquiring a cheaply made Chinese product.

It is difficult to overestimate the importance of the Pasdaran, as the guards are known in Iran. Mr Ahmadinejad served with them during the Iran-Iraq War, when they were at the forefront of the fighting. The unit reports directly to Ayatollah Ali Khamenei, controls large business interests and is heavily involved in Iranian politics.

Last month its prestige was challenged by reports that General Ali Resa Asgari had disappeared in Turkey and possibly defected to the US. Iran insisted that he was kidnapped, but more reports have since surfaced of senior officers disappearing. US and British commanders in Iraq have been on alert against the threat of Iranian retaliation. The opportunity presented itself on Friday when the two lightly armed Royal Navy inflatable boats were surrounded by a larger armed force of Revolutionary Guard naval patrol boats.

Before HMS Cornwall could respond, the 15 Britons were seen being taken to a small naval base on the Shatt al-Arab waterway, which marks the Iran-Iraq frontier.

Britain faced this predicament three years ago, when eight sailors and Marines were seized in similar circumstances by the Iranians. The men were freed unharmed after three days.

This time the process could be longer and more complicated.

Britain’s relations with Iran have deteriorated, in part because of London’s role in pushing through UN sanctions over Iran’s nuclear programme. Tensions have mounted in the Gulf, with a build-up of US naval power in recent months.

As for the five Iranian detainees, they are being held at a US prison camp in Iraq, and still being interrogated. There are no plans to free them while they are seen as a security risk in Iraq.