Amnesty International has called on Colonel Gaddafi to release four Al Jazeera journalists held for more than two weeks after being detained trying to leave Libya.
The international human rights charity said the two correspondents and two cameramen who were arrested in Zantan, near the Tunisian border, and could be at risk of torture.
The missing correspondents are Ahmad Val Wald-Eddin from Mauritania and a Tunisian, Lutfi Al-Massoudi - both 34 years old.
Norwegian cameraman Ammar Al-Hamdan, 34, has also been detained along with Ammar Al-Tallou from Britain.
It is thought that Lutfi Al-Massoudi may be held in Tripoli, after a CNN correspondent posted on Twitter that one of his colleagues had been detained with a Tunisian correspondent in the capital.
An Al Jazeera cameraman, Hassan Al Jaber, was killed in an ambush in Libya last week.
Malcolm Smart, Amnesty's Middle East and North African director, said: "This is the latest disturbing example of a campaign of attacks and harassment against journalists trying to do their job of covering the conflict in Libya.
"The Libyan authorities in Tripoli must reveal the whereabouts of these journalists, protect them from torture and release them immediately.
"Abuses against journalists seeking to report the facts are totally unacceptable and point to a deliberate effort to prevent the truth emerging through the fog of war."
Amnesty International said the Libyan authorities have targeted journalists working for foreign news organisations since unrest started in the country.
Experienced foreign correspondent Dave Clark, 38, is the latest British journalist to have gone missing. He has not made contact with his editors at the Agence France-Presse (AFP) news agency since Friday evening, the agency said last night.
Three BBC journalists were tortured and subjected to mock executions while being detrained by Gaddafi's forces two weeks ago.
The Libyan authorities also held Ghaith Abdul-Ahad, a journalist for the Guardian newspaper, for a fortnight before being freed last week.
Those authorities yesterday also released four journalists from the New York Times newspaper, six days after their arrest.