An investigation is continuing into the kidnap of a British aid worker in his 70s, along with two Canadians and a US citizen in Iraq.
Retired hospital professor Norman Kember opposed the Iraq war
Norman Kember, a retired professor from London representing several aid groups, was snatched from western Baghdad by an unknown group on Saturday.
Foreign Secretary Jack Straw said it was a "very, very difficult and worrying situation for the family".
On Monday, two Britons and an Iraqi were shot dead near Baghdad airport.
The pair and at least three other foreigners were shot on a minibus as they travelled towards the airport after a pilgrimage to holy sites.
Meanwhile, Germany's foreign ministry has said it is trying to find a German woman reportedly kidnapped in Iraq on Friday.
Images of her blindfolded and surrounded by her kidnappers, apparently taken from a video demanding Germany cease dealings with the Iraqi government, have been shown on German television.
Mr Kember's wife Pat is waiting for news of her husband at their home in Pinner, north-west London.
No contact has been made by his kidnappers since his abduction.
Mr Straw said officials had been in touch with Mr Kember's family and that he had discussed the matter with Iraqi Foreign Minister Hoshyar Zebari.
"We are working on the basis it is a kidnapping," Mr Straw said.
He added that the UK had a "clear and consistent" policy that it did not pay ransoms.
Canadian authorities have not named the two nationals seized along with Mr Kember, who opposed the Iraq war.
The British authorities say they are working closely with officials from the other countries involved.
A multi-national hostage team is thought to be focusing on who seized the four and what their motives were.
Canadian government spokesman Dan McTeague said it was working with an aid organisation to find out more about the kidnappings.
But he added: "One thing we will not do and we have never done is to negotiate with kidnappers."
So far few details have emerged of how the group was abducted or who may have been behind it.
But it is thought the westerners were in a district of western Baghdad with "pretty minimal" security when they were snatched.
The BBC's Jim Muir in Baghdad said the authorities had said very little since the group was taken in the hope that keeping a low profile would speed an early release.
However, the fact there had been no contact or ransom demand from the kidnappers suggested the motive for the crime may not be financial, he said.
Post-war Baghdad has seen many car bombings and kidnappings
Mr Kember used to be the secretary of a peace group in Pinner, although his wife said he was no longer with the group.
Mrs Kember said she had been well supported and that "people are being very, very good to me".
An official at one of the peace groups where Mr Kember sometimes works said he was unaware he had been in Iraq.
"I've known him since the 1960s. He's a peacemaker. He's a person of great resources... he'll keep his sense of humour.
"He's not a foolhardy man, he'd know exactly what he was getting himself into."
He said Mr Kember had been a professor at a teaching hospital in London.
He added: "He was incredibly active. He was against the war. He went on the big demonstration against it in London and felt very strongly about it.
"As far as I know, he went to Iraq as an individual, he didn't go as part of our group."
This is the first snatching of foreign civilians in Baghdad since the kidnapping of Guardian reporter Rory Carroll, who was held for 36 hours last month and later released unharmed.
Most Western aid agencies pulled their workers out of Iraq following the abduction of aid worker Margaret Hassan in 2004.
Few now operate outside Baghdad's heavily-guarded Green Zone.