"I think the whole country will want to send the families our very sincerest condolences.
"There is no justification for hostage-taking and we call upon those people who are holding the other Iraqi and British hostages to release them immediately."
Confirming the victims' identities, Miliband said: "Hostage-taking is never justified in any cause. Today's terrible news underlines the gravity of the crimes associated with it."
Graeme Moore, father of hostage Peter Moore, said on Saturday: "It's a real blow, particularly for the other two families.
"At least it gives us hope for Peter and whoever the other two are, if they are still alive."
The group which kidnapped the five men had sent a video featuring Moore to the British embassy in Baghdad, the British foreign ministry said in March.
Britain's Channel 4 News said the video showed Moore saying the group was being treated well.
More than a year earlier, another video featuring Moore had been aired by Dubai-based Al Arabiya television, in which he called on Gordon Brown, the British prime minister, to free nine Iraqis in return for their freedom.
The group had previously demanded that Britain withdraw its troops from Iraq.
In a video released in July, the group claimed that one of the men had committed suicide, but this has not been confirmed.