Amnesty International has called on the Libyan authorities to release four Al Jazeera journalists held incommunicado since they were detained while trying to leave the country two weeks ago.
The two correspondents and two cameramen were arrested in Zantan, near the Tunisian border, and could be at risk of torture.
An Al Jazeera cameraman, Hassan Al Jaber, was killed in an ambush in Libya last week.
"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," said Malcolm Smart, Amnesty International's Middle East and North African director.
"The Libyan authorities in Tripoli must reveal the whereabouts of these journalists, protect them from torture and release them immediately."
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.
Many journalists have been targeted during the unrest in Libya.
Three BBC journalists were tortured and subjected to mock executions while being detrained by Colonel Gaddafi's forces two weeks ago.
Ghaith Abdul-Ahad, a journalist for the UK's Guardian newspaper, was held by the authorities for a fortnight and held in solitary confinement before being freed last week.
The Libyan authorities today released four journalists from the New York Times newspaper, six days after their arrest.
"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," said Malcolm Smart.