Prime Minister Gordon Brown said on Saturday he was taking seriously a claim by militants in Iraq that one of five Britons they are holding hostage has killed himself.

The Sunday Times reported that the kidnappers made the claim in a videotaped statement handed to its representative in Iraq last week.

The statement said one of the hostages, named as Jason, committed suicide on May 25 -- one year after the Britons were seized by a Shi'ite militant group from inside an Iraqi Finance Ministry building in a raid in Baghdad.

"Clearly this is a very distressing development. We are taking this very seriously. There are many people working behind the scenes trying to find a solution," Brown said in a statement.

Brown said he discussed the issue of the five hostages with Iraqi Prime Minister Nouri al-Maliki in Baghdad on Saturday. "We both share a desire to see them returned safely to their families. I call on those holding the hostages to release them immediately and unconditionally".

A Foreign Office spokesman in London said: "There is no independent verification of the claims in this video, therefore we are not going to comment on its veracity".

Iraq's national security adviser, Mowaffak al-Rubaie, had said in June that the five Britons, who have not been named, were still alive.

There was no news of the men until a video featuring one of them was released last December. It included a statement from the captors threatening to kill him unless Britain pulled its troops out of Iraq.

A second video was released in February, this time featuring a second hostage who appealed for the release of nine Iraqis in return for the Britons' freedom.

The Sunday Times said that another hostage, named as Alan, appeared in the video appealing for the British government to hasten the men's release.

"Physically, I'm not doing well," he said. "Psychologically, I'm doing a lot worse. I want to see my family again."

"I would like for the British government to please hurry," says Alan, who looked pale. "Please hurry and try and get this resolved as soon as possible," he said.

The video, entitled "Intihar", Arabic for suicide, opens with a photograph of a man wearing a football shirt. He is identified as Jason in the statement, which appears on screen in Arabic and is signed, "The Shi'ite Islamic Resistance in Iraq".

The statement accuses the British government of responding indifferently to messages from the kidnappers and the captives.

"This procrastination and foot-dragging and lack of seriousness on the part of the British government has prolonged their psychological deterioration, pushing one of them, Jason, to commit suicide on 25/5/2008," the statement says.