US authorities have criticised Denmark for allowing a former Guantanamo prisoner to travel to join rebels in Syria, where he is thought to have been killed several weeks ago.

Slimane Hadj Abderrahmane, a Danish citizen, was captured by the Americans in Afghanistan in 2001 after attending a military training camp and was transferred to the US military base in Guantanamo, Cuba.

Now the American negotiator who helped secure Abderrahmane's release in 2004 has criticised the Danish security services for failing to ensure that he did not return to fighting

“The point of the deal was that Denmark would ensure that the prisoner did not return to the battlefield,” Pierre-Richard Prosper told public broadcaster DR.

While the US considered Abderrahmane to be a threat because he had attended a military training camp and had close ties to known terrorists and Islamic extremists, Danish authorities refused to imprison Abderrahmane upon his return as they argued he had not broken any laws.

After a year of negotiations, the two sides reached a compromise in which Denmark promised to use the social services, police and intelligence agencies to keep a close eye on Abderrahmane once he returned to Denmark.

The goal was to ensure that Abderrahmane did not return to fighting, which is exactly what seems to have happened.

“The main requirement was that the Danish government needed to control him and our goal was to ensure he did not return to the battlefield,” Prosper said. “What we know is he returned to jihad and that’s a problem.”

There has yet to be any official confirmation of Abderrahmane’s death. Various media reports suggest, however, that he was killed while fighting with the Al-Nusra Front in Syria, which was designated as a terrorist organisation by the US last December. Another Danish man, Abdul Malik, was also recently reported to have been killed while fighting in Syria.

Prosper argued that Abderrahmane’s participation in fighting in Syria meant that the Danish security services let down their guard.

“Somewhere along the way the system became complacent and saw him as a non-issue,” Prosper said. “One has to wonder whether or not there was a failure in intelligence leading up to getting his passport because one has to assume that he had to communicate with someone to know where to go and know what the nature of his fight in Syria would be.”

In a comment to DR, the domestic intelligence agency PET said that Abderrahmane's freedom of movement was not limited by the agreement that Denmark signed with the US and that the US was informed of his trip to Syria.

"This handover of information did not result in any comments from the US as far as the release of Slimane Hadj Abderrahmane from the Guantanamo base is concerned," PET said.