Human Rights Watch on Monday called on US Secretary of State John Kerry, who visits Ethiopia on Tuesday, to urge Ethiopian officials to release the nine.
The group's Leslie Lefkow said the arrests again signal that anyone who criticizes the Ethiopian government will be silenced.
Amnesty International noted that 20 members of a political opposition group were also arrested last week. Amnesty said Ethiopia is tightening "its suffocating grip on freedom of expression".
The US State Department's report on Ethiopia's human rights practices in 2013 found that authorities continued to arrest, harass and prosecute journalists and other citizens for criticizing the government.