"They are suspected of some serious crimes, and the police are investigating the case," government spokesperson Getachew Reda told AFP, without providing details of their alleged crimes.
The journalists and a group of bloggers known as "Zone 9" were arrested last week, prompting an outcry from rights groups.
The Committee to Protect Journalists (CPJ) called the arrests "one of the worst crackdowns against free expression" in the country, while Amnesty International said it was part of a "long trend of arrests and harassment of human rights defenders, activists, journalists and political opponents".
Leslie Lefkow of Human Rights Watch said the "arrests signal, once again, that anyone who criticises the Ethiopian government will be silenced", and called for their immediate release.
The bloggers Zone 9 website, reportedly named after the prison where political detainees are held, listed the names of nine people arrested, saying they were charged with having worked with foreign human rights activists to foment violence or instability.
But the government dismissed the rights groups, and said those arrested were not detained for their work as journalists.
"We don't take orders from Human Rights Watch," Getachew said.
An opposition group staged a protest on Sunday following the arrests, calling for "greater liberties and a true democracy" in Ethiopia, but police shut the 200-person demonstration down soon after it started.
HRW said 20 members of the political opposition Semayawi or "Blue" party have also been arrested since Friday, although there has been no official confirmation of exact numbers.