They appeared, somewhat diffidently, from behind a traditional thatched mud and wattle hut, about 30 of them in all. The forgotten prisoners of a forgotten war, abandoned by their own government.

Some smiled, others looked a little wary. They all shook my hand and then sat down in three straight lines.

This is a prison camp like no other. These men are held captive by the Sudan People's Liberation Movement-North, (SPLM-N), the rebel movement which has, for five years, been fighting the forces of Sudan's President Omar al-Bashir in the Nuba Mountains of South Kordofan.

Some of the men have been held here for four years. But what makes this situation so unusual is that their captors are trying to hand them back to their government in Khartoum. But Khartoum, it seems, doesn't want them. Indeed, it appears to be denying all knowledge of them.

Although everyone here calls them "prisoners of war", this is actually a civil conflict so they are, technically, detainees.

The most senior officer among the captives is a quiet dignified man, Brigadier General Refaut Abdalla Ahmad. He has the manner of a gentle college professor, although his captors tell me he was a tough soldier, respected by his erstwhile enemy.

He admits that he has heard that Khartoum has denied all knowledge of him: "The government of Khartoum says that it has no brigadier called Refaut Abdullah here, who was captured," he says. "I am not here."

But with a soldier's loyalty, he will not be drawn on how that makes him feel. "I am a military man … The government knows what they do. I accept anything done with me. "

Last June, there was hope of a breakthrough when it was arranged for 12 prisoners from this group (there are 86 held here in total), to join 10 prisoners held by the SPLM-N in the Blue Nile to the east, in a transfer facilitated by the International Committee of the Red Cross (ICRC).

The 12 prisoners in the first group were taken to Kauda, the rebel's administrative capital where a disused dirt runway was cleared so a Red Cross plane could pick the prisoners up and transport them to Ethiopia for an onward flight to Khartoum.

But it was not to be. For four days they waited in Kauda - but eventually, they were told the transfer was not going to happen.

For the commander of the rebel army, Major General Jagod Marada, it is clear where the blame lies.

"At the very last moment, the Sudanese government blocked all the agreements," he said, claiming that they did so partly because they have already declared the prisoners dead - and informed their families - and partly because they don't have any prisoners themselves to exchange.

"The government don't have our prisoners in their hands to exchange because they killed most of them … They don't respect international laws and humanitarian conventions," he said.

A Sudanese government spokesman said the SPLM-N had "no grounds" for their claims, dismissing them as "fabricated allegations".

The ICRC, in line with its policy of strict neutrality, would not comment on where the blame lies, but said the prisoner transfer had been "postponed", adding: "The ICRC calls on all parties involved to continue working together to allow the concerned detainees to return home to their families as soon as possible."

For the 12 men who had been told they were to return home, the failure of the operation was devastating.

"After that, the dream was destroyed," said one of the men, Alpidoney Babeker. "This is so, so bad for us. Many people in this group are young boys. Their hearts are broken, but what can we do?"