(CNN) -- U.N. officials said they will toughen efforts to thwart the kind of mass rape reported recently in the Democratic Republic of the Congo, and they urged the country to ramp up a search for a missing human rights activist believed to be kidnapped.
Both the abduction and the rapes occurred in North Kivu in eastern Congo -- where militia groups have slugged it out among themselves and government forces for years.
Margot Wallstrom, the U.N. secretary general's special representative on sexual violence in conflict, said the United Nations is "currently pursuing a two-pronged approach" in dealing with the rape problem.
"Firstly we must hold the perpetrators to account. The illegal armed groups that continue to rape as a weapon of war must be brought to justice. And secondly we must improve the U.N.'s response," she said.
The United Nations said at least 154 civilians were raped in 13 villages in North Kivu province's Banamukira territory between July 30 and August 2.
The Mai-Mai militia and the Democratic Forces for the Liberation of Rwanda, a group of ethnic Hutu fighters linked to the 1994 genocide in Rwanda, have been blamed.
Wallstrom said rapists who remain free "hold the whole reputation of the Congo hostage."
"The latest atrocities reinforce a key finding from my mission: You cannot have a policy of zero tolerance backed by zero consequences," Wallstrom said.
Also, the U.N. peacekeeping force in the Democratic Republic of the Congo urged the government to find a well-known activist who has gone missing and called on Congo officials to "stop the threats, intimidation and other acts of harassment of human rights defenders throughout the entire DRC."
Sylvestre Bwira Kyahi, the president of the Civil Society of Masisi, was last seen on August 24 in Goma, the capital of North Kivu province.
Kyahi, whose group is a nongovernmental organization, has spoken out against alleged rights violations by the country's military.
The U.N. peacekeeping force in the Democratic Republic of the Congo said two men clad in military uniform reportedly made Kyahi "board a military vehicle." He has been threatened and harassed in the past, particularly after he issued an open letter last month accusing soldiers in the Masisi region of rights violations.