A message claiming to come from a group led by the Islamic militant Abu Musab al-Zarqawi has called for the release of charity worker Margaret Hassan.
Mrs Hassan has lived in Iraq for 30 years and married an Iraqi
Her kidnappers had been threatening to hand her to Jordanian-born Zarqawi's group unless British troops quit Iraq.
The message promises to free Mrs Hassan if she falls into their hands.
The message, signed "al-Qaeda in Iraq", was posted on a website known for publishing messages from Islamic militant groups.
It urges Mrs Hassan's kidnappers to publish any evidence against her and calls for her immediate release unless she is proven to be a spy "conspiring against Muslims".
"These people who are using this prisoner as a playing card didn't know our religion very well," it adds.
"In true Islam, they don't kill women and young children.
"We never kill people who we are not supposed to kill.
"We only kill those who fight us and kill our people.
"Many times we stop operating a successful attack because we know that Muslim people would be killed.
"We cancel these operations to save the blood of Muslims."
The message was posted under the pseudonym Abu Maysara al-Iraqi - the name usually associated with statements from al-Zarqawi's group, believed to number up to 500.
The group, formerly known as Tawhid and Jihad, says it beheaded several Western hostages, including American businessman Nick Berg and construction worker Ken Bigley.
It also says it carried out some of Iraq's bloodiest suicide attacks and vehicle bombings, including last year's attack on the UN's Baghdad headquarters and the suicide bomb that killed three Black Watch soldiers on Thursday.
Dublin-born Mrs Hassan, who has Irish, British and Iraqi nationality, was seized by an unknown group in Baghdad on 19 October.
She was driving to work as director of Care's Iraq operations. The agency has since halted work in the country.
The 59-year-old, who has lived in Iraq for 30 years and is married to an Iraqi, has appeared in several videos since her kidnapping.
In them she has given statements calling for Britain to withdraw troops from Iraq, and for women prisoners to be freed.