The wife of an American taken hostage with two others in Baghdad has appealed on television for their release.
The hostage takers want women prisoners freed
Patty Hensley said her husband Jack was "a simple, generous man" who was "there to help the Iraqi people".
Militants have threatened to kill Mr Hensley, American Eugene Armstrong and Briton Kenneth Bigley unless all female Iraqi prisoners are freed.
The captors, reportedly supporters of al-Qaeda militant Abu Musab al-Zarqawi, issued their demand on a video tape.
Arabic TV station al-Jazeera has meanwhile broadcast another tape in which unknown militants threatened to kill 10 hostages unless their US-Turkish company left Iraq.
The militants - from a group called the Salafist Brigades of Abu Bakr al-Seddiq - gave the unnamed company three days to withdraw from Iraq.
In other developments:
- A videotape posted on an Islamic militant website shows the apparent beheading of three Iraqi members of the Kurdistan Democratic Party, which co-operates with the Iraqi government. Its authenticity could not be verified
- Iraqi security forces conducting a raid near Nasiriya free a Jordanian civil servant who was kidnapped last month
- Four people are reported killed and five wounded in Falluja, after a US air strike on what it said was an illegal militant checkpoint
- A roadside bomb in central Baghdad kills one Iraqi man, police say
- A car bomb near the rebel stronghold of Samarra north of Baghdad kills at least one Iraqi and wounds at least three US soldiers, the US military say
Mrs Hensley was dressed in black as she made her sombre appeal on CNN TV.
"We ask for your mercy in freeing Jack and his co-workers so that they can continue to return home to their loving families, and it is your decision whether this happens," she said.
Mr Blair said he was doing everything possible to free the Western hostages.
The British embassy in Baghdad says it is following various lines of inquiry, and a telephone hotline has been set up in Iraq for local people to call with any information.
The group said to be holding the hostages is Tawhid and Jihad (Unification and Holy War).
Responding to its demand, the US government said it is holding only two women in Iraq - high-level detainees who are understood to have been part of Saddam Hussein's regime.
In London, Iraqi foreign minister Hoshyar Zebari said his government would not bow to the captors' demands.
"It would set a very
bad precedent, and really our policy is not to negotiate with the terrorists," he told the BBC's Breakfast with Frost TV programme.
Fighters waging a 17-month insurgency in Iraq have kidnapped more than 100 foreigners in an effort to destabilise the interim government and drive foreigners from the country.
They include two French journalists who were abducted almost a month ago and two female Italian aid workers, along with two of their Iraqi colleagues, who were kidnapped on 7 September.
The Italian news agency Ansa quotes Deputy Iraqi Foreign Minister Hamid al-Bayati as saying that the two aid workers may have been sold by their kidnappers to members of Abu Musab al-Zarqawi's group.