Journalists are facing "unprecedented threats" in Egypt, a media watchdog said Thursday, with a record number behind bars mostly over links with the now-outlawed Muslim Brotherhood group.

The Committee to Protect Journalists (CPJ) said a prison census on 1 June showed that at least 18 Egyptian journalists, most of who worked for online media, were imprisoned — the highest number since it began recording data in 1990.

"The threat of imprisonment in Egypt is part of an atmosphere in which authorities pressure media outlets to censor critical voices and issue gag orders on sensitive topics," the CPJ said in a report published Thursday.

The New York-based group said that high government officials it spoke to, including the public prosecutor and the minister of transitional justice, denied journalists were locked up in association with their reporting. But its research found that the current government "has used the pretext of national security to crack down on human rights, including press freedom."

At least one more journalist — Mohamed Al-Batawy of the state-run Al-Akhbar newspaper — was arrested following the CPJ's research.

Gamal Abdel Rehim, under-secretary of Egypt's Press Syndicate, said the number of detained journalists could be higher. He added that imprisoned syndicate members alone are no less than 18, with all facing charges of links to the Brotherhood.

"Press freedom in Egypt is being trampled on everyday in an unprecedented way," Rehim told Ahram Online. The country is witnessing the worst phase in this regard, even worse than during Brotherhood rule and that of [long-time strongman] Hosni Mubarak."

Since the 2013 ouster of Islamist president Mohamed Morsi, Egyptian authorities have waged a crackdown on the Muslim Brotherhood and other secular activists, which have seen hundreds killed and thousand jailed. An Islamist insurgency has spread since, with deadly attacks targeting security forces.

The media rights group noted that some media outlets, including the Qatar-based Al-Jazeera network and the Turkish Anadolu News Agency, have been banned from operating or forced shut down their offices.

The government had "indiscriminately charged" journalists and political detainees belonging to the blacklisted Brotherhood, it added.

Six journalists were sentenced to life in prison while several others are being held in pre-trial detention with no date set for a court hearing, the CPJ reported.

"The arrests of journalists in Egypt are often violent and involve beatings, abuse, and raids of their homes and confiscation of their property," the CPJ said.

"Their prison cells are often unclean and overcrowded. In letters from prison, some journalists wrote that they often do not see sunlight for weeks; others described the torture of prisoners, including the use of electric shocks."

Egyptian officials have repeatedly denied allegations of torture in Egypt's jails.

The CPJ said the heavy restrictions on journalists have left entire regions "severely underreported, most notably in the restive Sinai Peninsula," a bastion of a burgeoning Islamist militancy.

It said scant information is known about the battle in the Peninsula between troops and militants there, or the conflict's toll on civilians.

"Coverage there is almost totally banned, and the only reporting we do is to publish government statements," Abdel Rehim said.

"But still that could be for security reasons or for fear for the lives of journalists."