Campaign group Reporters Without Borders called Monday for the immediate release of 12 journalists detained in a wave of arrests in Ethiopia's capital Addis Ababa.

Most of the journalists, including one who runs a YouTube channel critical of the government, were arrested on June 30, according to Reporters Without Borders, known by its French acronym RSF.

It said no explanation had been given for their arrests until July 2, when police linked the journalists to a terrorist group, referring to the Tigray People's Liberation Front (TPLF).

Federal troops have been battling TPLF forces in Tigray since November, but they withdrew at end of June in the face of rebel advances and Prime Minister Abiy Ahmed's government then declared a unilateral ceasefire.

Ethiopia's government has imposed tough restrictions on journalists covering the conflict.

"We condemn these mass arrests of journalists, which clearly aim to deter independent investigative reporting on the war in Tigray," said Arnaud Froger, the head of RSF's Africa desk.

"These arrests, conducted in a completely opaque manner, are all the more shocking because, just a few months ago, the Ethiopian parliament voted a new media law decriminalising most press offences," added Froger, calling for their release.

The Ethiopian Human Rights Commission at the weekend voiced "grave" concern at the arrests, saying "the detainees have not been granted visitation rights by their lawyer or their families."

A foreign correspondent working for The New York Times, Simon Marks, was expelled from Ethiopia in May after he filed a series of hard-hitting reports on the war in Ethiopia's northern Tigray region.

A number of Ethiopian journalists and translators working for a range of international media organisations -- including AFP, Reuters, the BBC and the Financial Times -- have been detained in recent months while doing their jobs.

Reporters Without Borders ranks the Horn of Africa nation at 101 out of 180 in the world on its annual press freedom index.