Sudan Armed Forces spokesman Sawarmy Khaled said the pilots, working for the private Badr Company, were kidnapped Sunday in Nyala, the capital of South Sudan province.

The army has surrounded the neighborhood where the kidnappers are beleived to be holding the men, Khaled said.

Al-Sahafa newspaper on Monday, quoting unnamed sources in Nyala, said three pilots were kidnapped, rather than two.

The paper added that the men worked for a company transporting food for the peacekeepers.

Sudan's Foreign Ministry, meanwhile, said a US aid worker kidnapped four months ago has been freed by police.

Ministry spokesman Muawiya Khaled told The Associated Press Monday that a woman working with the North Carolina-based Samaritan's Purse organization was freed by police in South Sudan province and will be back in the capital Khartoum soon.

A 36-year-old California woman and two Sudanese working for an aid organization run by evangelist Franklin Graham were kidnapped by armed men in May.

Samaritan's Purse says it has provided $83.7 million in assistance to Sudan over the past decade.

Violence had largely subsided in Darfur over the last year, but a recent spike followed the April elections, including a number of kidnappings.