The main target of the strike, Abu Ali al-Harithi, is said to have been involved in the October 2000 attack on the USS Cole in Aden harbour that killed 17 US sailors.

Al-Harithi has been sought by US and Yemeni authorities since 11 September 2001, and was sheltering with Yemeni tribesmen in the north of the country, which is largely outside government control. A Yemeni military attempt to apprehend him last November led to the deaths of about 20 soldiers in a firefight with local tribesmen.

The missile attack, which was carried out by an unmanned Predator drone aircraft, was described by US Defence secretary Donald Rumsfeld as 'a very successful tactical operation' that 'imposed changes in [Al-Qaeda] tactics, operations and procedures'.

The use of targeted assassinations outside Afghanistan marks a new phase in the US-led campaign against international terrorism. It was carried out under new procedures approved by US President Bush that give the CIA and US military authority to conduct covert international operations with the consent or knowledge of host governments.

However, the administration denied the operation violated its own ban on assassinations, saying the regulation only applied to those who were recognised as political leaders.

Yemen has worked to improve its security image since 11 September, when it was widely seen in the West as a haven for Islamist terrorists. US special forces have been training the Yemeni military since early this year in counter-terrorism techniques, to allow them to pursue suspects alone.

'We call on everyone from among our countrymen who have been entangled in membership of the Al-Qaeda organisation to repent.and renounce all means of violence,' said a statement by President Saleh.