Kidnappers of Irish-born Margaret Hassan issued a 48-hour deadline for British troops to leave Iraq or they would hand her over to a group led by Abu Musab al-Zarqawi, blamed for a string of hostage beheadings.

Her employer, CARE Australia, urged the militants to free the 59-year-old Iraqi citizen following the release of a new video showing her in a deeply distressed state as she pleaded for her life.

In the latest abductions, five Jordanians, most of them truck drivers, were reported kidnapped while a US-Lebanese contractor was seized at gunpoint from an upmarket district in Baghdad.

In other developments, three headless bodies have been found beneath a bridge on a road across the Tigris river that leads into Baghdad’s fortified Green Zone.

It was not clear if they were the bodies of Iraqis or of any of the two dozen foreign hostages missing in Iraq.

Al Jazeera television earlier reported a militant group had beheaded three Iraqi National Guards.

The Arabic TV station aired video footage from the previously unknown “Brigades of the Iraqi Honourables”, which showed three men with a masked militant standing behind them.

The television station said it would not show them being beheaded out of “respect to viewers’ feelings”.

Elsewhere, a US soldier was killed by a roadside bomb and fires raged near oil wells after a string of sabotage attacks.

US and Iraqi officials are building up for an anticipated all-out offensive on the restive cities of Fallujah and nearby Ramadi.

US war planes unleashed two overnight raids on Fallujah, west of Baghdad, but it was not immediately clear if there were casualties.

US ground troops have encircled Fallujah since mid-October, and interim Prime Minister Iyad Allawi issued an ultimatum to the city on Sunday to surrender insurgents holed up inside or face an all-out military assault.