The United Nations has scaled back its humanitarian effort at Kenya's sprawling Dadaab refugee complex, as government troops search for two kidnapped aid workers.

U.N. officials say all non-lifesaving activities have been put on hold until the security situation improves at the world's largest refugee camp.

Aid groups are continuing to provide food, water and medical services to nearly half-a-million people at the camp. Many fled from Somalia to escape drought, famine and insecurity.

Kenyan police believe the two European women were abducted by Somalia-based al-Shabab militants, a charge leaders with the al-Qaida-linked group have denied. No ransom demands have been reported.

Security officials searched for the kidnapped workers by land and air near the border on Friday. Authorities discovered their hijacked vehicle late Thursday, about 50 kilometers away from the camp toward the Somalia border.

The medical aid group Doctors Without Borders has confirmed two Spanish staff members were kidnapped at Dadaab. The group announced Friday it has pulled its foreign staff members from the camp.

It is the third kidnapping of foreigners near the Somalia-Kenya border in the past month.

A British tourist and a French woman were recently abducted at Kenyan resorts. Both of those women are being held captive in Somalia.