The reward offered by the US was initially set at $25 million. This made al-Zarqawi one of only five men for whom the US State Department’s Rewards for Justice programme offered such a high bounty, which was posted on Tuesday 28 October.

But on Thursday, US media noticed the reward had been quietly downgraded to $5 million. Almost two dozen figures are on that less exclusive list.

The reason for the change remains unclear. The posting on the Rewards for Justice website offered no explanation, not even a notice that it had changed.

A spokesman for the State Department told MSNBC.com only that "officials were looking into the amount that could be paid".

Yet by Friday, the $25 million reward reappeared with no explanation or notice of the change, the US news network reported. A check of the website on Saturday, however, showed al-Zarqawi's bounty was mysteriously back down to $5million.

Aljazeera.net on Saturday rang the Rewards for Justice programme telephone number assigned to receive information on dangerous international terrorists - but there was no answer.

Jordanian fugitive

The US accuses al-Zarqawi – a Jordanian fugitive with alleged close ties to al-Qaida - with recruiting resistance fighters in Iraq.

"Abu Musab al-Zarqawi has had a long standing connection to senior al-Qaida leadership and appears to be highly regarded among al-Qaida and a close associate of Usama bin Ladin," a US State Department announcement said.

The announcement for the reward came on a day thought to be al-Zarqawi's 38th birthday.

The one-legged Zarqawi has also been accused by Jordan of plotting the murder of a US diplomat Laurence Foley in Amman last October. Foley was gunned down before his home in the Jordanian capital.

Zarqawi is thought to be in Iraq, where his mission is to form a new network of resistance fighters.