The arrest and three-day imprisonment of eight UK Royal Navy personnel who strayed into Iranian waters in the Shatt al-Arab waterway created new tension between London and Tehran in mid-June, even though the men were eventually freed on 24 June. The incident reflects a growing assertiveness in the Islamic Revolutionary Guards Corps (IRGC).

The UK Ministry of Defence says the men were captured after their three patrol boats strayed deep inside Iranian territory early on 21 June after losing their way in poor weather. Iran's state-run Arabic-language satellite channel Al-Alam showed footage of the men blindfolded walking in single file along a beach, apparently soon after their arrest.

Two of the sailors apologised for their mistake on Iranian television. UK diplomats on 23 June travelled to Mahshahr, near the Bandar Imam petrochemicals complex in Khouzestan, to negotiate their release. Foreign Affairs Minister Kamal Kharrazi and UK Foreign Secretary Jack Straw had discussed the incident by telephone and Kharrazi promised to look into it personally. On 23 June he said it was clear the men had not crossed the border with 'ill intent' and said they would be released. The following day they were finally set free and flown to the UK embassy in Tehran.

The release of boats and equipment impounded with the Royal Navy crews is still under negotiation. London says the equipment was being delivered to Iraqi river police for use in anti-smuggling operations.

IRGC forces have recently become more assertive in the Gulf. In June, a series of incidents led to the capture of eight UAE fishing vessels that had allegedly violated Iranian waters. Iranian ships have been seized after entering Qatari and UAE waters.

Maintaining the Shatt al-Arab border has long been sensitive for Iran. The 1980-88 Gulf war with Iraq was fought in part to defend the border after Saddam Hussein ripped up the 1975 Algiers Treaty delineating the line as running along the deepest part of the channel. Baghdad has traditionally claimed ownership of the whole waterway.

The decision to arrest the men, and the later threats that they would be prosecuted, suggest the IRGC is sending a tough message both to its neighbours and domestic opponents. Hardline political allies of the corps' leadership remain deeply opposed to the presence of UK and US forces in Iraq. And the IRGC is apparently increasing its political involvement, pushing its candidates for powerful positions and raising its physical presence in towns and cities.

The UK has recently incurred the anger of hardliners by helping draft a resolution from the International Atomic Energy Agency that was critical of Iran's nuclear programme. London was also active in the EU's harsh rebuke of the Islamic republic's human rights record. Conservatives in May demonstrated outside the UK embassy in Tehran on consecutive days, throwing stones and homemade explosives.