Three of its peacekeepers were killed and a fourth seriously wounded Monday when 20 assailants opened fire in an ambush in Sudan, The United Nations has said.

The U. N. News Center has reported that the African Union-United Nations peacekeeping force in Darfur promised it would not be intimidated from carrying out its work despite the 11:30 a. m. attack on the Rwandan soldiers in the West Darfur village of Nertiti.

The office of U. N. Secretary-General Ban Ki-moon issued a statement in New York saying the Sudanese government must make every effort to apprehend the assailants.

The statement said, "The secretary-general expresses his condolences to the families of the peacekeepers who lost their lives and to the government of Rwanda and reiterates his appreciation for their service and commitment to the search for peace in Darfur."

According to the United Nations, the killings bring to 27 the death toll in the Darfur region since U. N. peacekeepers began their mission in January 2008.

Since fighting began in 2003 in Sudan's arid west flank of Darfur, about 300,000 people are estimated to have been killed and another 2.7 million others displaced from their homes.

The United Nations further said that Militiamen, government forces and rebel groups have all been accused of human rights abuses. (With Inputs from Agencies)