A former Lebanese security chief detained over the murder of ex-premier Rafiq Hariri is suing a former U.N. investigator in the case, his lawyer said on Wednesday.

General Jamil Sayyed, the former head of the general security department, filed the lawsuit in France against Detlev Mehlis, attorney Akram Azouri told reporters.

Sayyed is among four officers, who are believed to be close to Syria, who are in custody in Lebanon over the February 2005 assassination of Hariri in a massive Beirut car bombing.

"Jamil Sayyed's lawyers in France have filed the lawsuit against Mehlis for distorting the investigation and calling false witnesses," Azouri told a news conference.

Sayyed and the other officers -- former presidential guard chief Mustafa Hamdan, the former head of the internal security forces Ali Hajj and the former chief of army intelligence Raymond Azar -- have been held without charge since August 2005 but all have protested their innocence.

Earlier this month prosecutor Saqr Saqr turned down a request to free the generals and in April the government defended their detention saying it was "perfectly legal."

Mehlis had implicated senior officials from Syria, which was the power broker in Lebanon for three decades until it withdrew its troops two months after Hariri's assassination.

But Damascus has strongly denied any involvement in the bombing.(AFP)