The British group was snatched by up to 40 men, some dressed in police uniforms, from the finance ministry in Baghdad yesterday morning.
Prime Minister Tony Blair, speaking from Libya last night, said his government would do "everything we possibly can."
Earlier, a Foreign Office spokeswoman said embassy officials were in "urgent" contact with the Iraqi authorities in a bid to resolve the crisis. Four of the kidnap victims are security professionals working for the firm GardaWorld.
The fifth is an expert who was working for the US management consultancy firm BearingPoint. A spokeswoman for the firm confirmed four security professionals and one of its clients were taken forcibly from a work site.
Reports said up to 40 police vehicles were seen at the scene of the kidnapping, sparking rumours that renegade officers were involved. Many of the officers were said to be wearing the camouflaged uniforms of Iraqi special police commandos.
Eyewitnesses reported seeing them block off either end of the street outside the ministry, which is not in the heavily-guarded Green Zone, before passing guards stationed at the ministry.