Three Christian aid workers have been kidnapped in Sudan after being ambushed by armed men, Samaritan's Purse, the Christian charity run by Rev. Franklin Graham, reported Wednesday.

Two Sudanese men and a 36-year-old woman from California were abducted Tuesday evening, May 18.

The staff members were traveling in a two-vehicle convoy when they were stopped by gunmen in Sudan's Darfur region. The attackers took the cars and three aid workers, leaving three other employees behind.

"I have visited Darfur several times and it is one of the most difficult places in the world to work," Graham said. "I'm asking for the prayers from people around the world for a quick resolution and the safe return of our Samaritan's Purse staff."

Tuesday's abduction comes one month after a similar incident with four South Africans from the U.N.-African Union peacekeeping mission in Nyala. The Sudanese government eventually negotiated with the captors and the workers were released.

Other aid groups like the Christian agency World Vision International have also been ambushed previously in the Darfur region.

Samaritan's Purse has provided nearly $84 million in relief assistance to Sudan since 2001 -- including medical supplies, food, clean water and rebuilding churches in the country's civil war.