Norman Kember in the latest video

The new video, aired on al-Jazeera TV, includes a message from the group, which calls itself the Swords of Truth, demanding all Iraqi prisoners be freed.

It says the four hostages will be killed if its demands are not met.

The tape shows the four hostages standing against a wall and is dated 21 January.

It is completely inconceivable that all the people in detention in Iraq would be released Nicholas Witchell

The men appear to be speaking to the camera but their words are not audible.

Mr Kember's fellow captives are Canadians James Loney, 41, and Harmeet Singh Sooden, 32, and American Tom Fox, 54.

Al-Jazeera said it had aired the whole of the recording, which was about 55 seconds long.

The Qatar-based satellite channel declined to reveal how it got hold of the tape.

A message on the tape demanded the US and Iraqi authorities release all their Iraqi prisoners.

The British Embassy in Baghdad said it was assessing the video of the hostages.

In a statement, it said: "After so long without word it is a relief that the video shows them alive. We continue to urge those holding them to release them."

We're extremely concerned and extremely worried about how they're looking and how they're dealing with the ordeal Chris Cole

Anas Altikriti of the Muslim Association of Britain, who has been campaigning in the Middle East for Mr Kember's release, said the video showed there had been "some kind of breakthrough" in the efforts to communicate with the captors.

He said he still did not know who was holding the men or where they were, but the video was a positive sign that they were still being held by the same group.

He told BBC News: "One of the things we were extremely concerned about over the last six weeks was that the initial group... would change hands and in a sense sell them off to a bigger, possibly even more ruthless group.

"Now I think we have reason to be hopeful."

Mr Altikriti said: "The contact after all this time... shows they are willing to negotiate, willing to talk and that is a very good thing. Otherwise they would have carried out their threat regardless."

Relief

But BBC correspondent Nicholas Witchell, in Baghdad, said it was "disturbing" that the group was still making political demands which could not be met.

He said the US had released 420 prisoners, including five women, two days ago but there were many thousands of people still in detention.

He added: "It is completely inconceivable that all the people in detention in Iraq would be released."

A friend of Mr Kember, Chris Cole, said he was relieved to hear news "after such a long period of silence".

"But obviously we're extremely concerned and extremely worried about how they're looking and how they're dealing with the ordeal," he told BBC News.

James Loney is one of two Canadians held hostage

He said he was sure that there was dialogue between the hostages and their captors.

"Norman's a man of great faith and we very much hope that they're all together - that will keep up their sprits.

"But, obviously, Norman's 74 - it's a very difficult time and we hope and pray that he remains strong."

Another friend, Sue Claydon, told Sky News it was "wonderful" to see evidence Mr Kember was still alive after so long without news.

The Reverend Alan Betteridge, said he was pleased to see his friend Mr Kember was still alive but was concerned the kidnappers were still making the same demands "couched in these threatening terms".

He said: "I had thought with all the pressure put on them from the Muslim world they would have found a way of releasing them without making these sorts of threats."

Mr Kember's wife, Pat, was "coping very bravely and strongly", he added.

Blindfolded

"I am sure it is with mixed emotions. But she has a very strong belief that non-violent peacemaking is a thing to be done and that goodness will overcome evil.

"She is very supportive of what Norman was doing in Iraq."

Mr Kember, a retired professor, was last seen in a video aired on al-Jazeera in early December, blindfolded and with his hands tied.

The Swords of Truth group had threatened to kill the hostages by 10 December unless Iraqi prisoners were released.

The deadline was then extended by two days and expired on 12 December. There had been no news of the hostages since then.

The group had travelled to Iraq with Canadian-based international peace group Christian Peacemaker Teams as a "gesture of solidarity".