BEIRUT (AFP) — Human Rights Watch (HRW) called on Monday for the release of a man held by Syria without charge since 2005 in connection with the murder of Lebanese ex-premier Rafiq Hariri.

"The Syrian authorities have been holding Ziad Ramadan for almost four years," the New York-based watchdog's Middle East director Sarah Leah Whitson said.

"If they have nothing to hide in the Hariri investigation, then they should immediately free him or charge him with a recognisable crime," she said in a statement.

The UN-backed investigation into the Hariri murder questioned Ramadan, a Syrian citizen working in Lebanon, when television channels broadcast footage of a man named Ahmed Abu Adas claiming responsibility for the Valentine's Day bombing that killed the former premier.

"Ziad Ramadan was a friend of Abu Adas and his colleague in a computer company in Beirut -- that's basically the link," the senior researcher at HRW's Beirut office, Nadim Houry, said.

The UN-backed tribunal has said officially that it never requested that he be detained, Houry added.

"What they were asking about was Ahmed Abu Adas: did he drive and did he have internet access," he told AFP.

After being questioned in Lebanon, Ramadan left for Syria where he turned himself in upon hearing they were looking for him, HRW said.

He was then detained for almost a year in a Syrian military security bureau, dubbed the Palestine branch, before being transferred back to the main prison in the city of Homs, north of Damascus.

In August 2007, Syrian authorities again transferred Ramadan to the Palestine branch without informing his family, who were able to see him just once, in September 2007.

His family has heard nothing of him since, Houry said.

"Our concern is twofold: the first is that he has been incommunicado and has for all intents and purposes disappeared since 2007," he told AFP.

"The second is if they have evidence that he committed a crime, then they should turn him in to court for a fair trial."

In May, the UN-backed Special Tribunal for Lebanon ordered the release of four Lebanese generals detained in Lebanon since 2005 in connection with the murder.

There are no other suspects being held by the tribunal in connection with the case.