A Medecins Sans Frontieres (MSF)
A Medecins Sans Frontieres (MSF) worker ushers newly arrived Somali refugees for polio vaccination at the refugee camp in Dadaab, near the Kenya-Somalia border, August 1, 2011. REUTERS

The U.N. refugee agency (UNHCR) said on Friday it had suspended non-critical operations at Kenya's Dadaab refugee camp as security forces scoured the region for two Spanish aid workers kidnapped from there a day earlier.

Some aid agencies have become increasingly concerned by the worsening insecurity at the sprawling camp where refugee numbers have swollen to more than 460,000 this year as famine and conflict drive Somalis across the border.

In spite of the mounting security fears, the brazen nature of Thursday's broad daylight attack on two women working for Medecins Sans Frontieres in the heart of the camp took relief workers by surprise.

Today we have to hold everyone (at) base. We just have a thin staffing that is going down to the camps with a police patrol because some services cannot be suspended, said Emmanuel Nyabera, spokesman for UNHCR Kenya.

That meant there would be no registration of new arrivals, he said, but that water distribution, for example, would continue.

It was not clear when the agency's staff would be able to return to normal operations, Nyabera said. For now there were no plans for an evacuation of foreign staff, he added.

The African Union strongly condemned the abduction, which came weeks after gunmen with close ties to Somali militants kidnapped two Western visitors from northern Kenya's coast in separate incidents.

Kenyan security forces fanned out through the semi-arid border area on Friday, hunting the abductors believed by Kenyan police to be al Shabaab rebels. The al Qaeda-linked rebel movement has denied it was responsible for the attack.

Kenya's North Eastern Province police commander Leo Nyongesa said the hostages whereabouts remained unknown.

We have contacted elders in Somalia to help us with the search, he said.

The MSF workers' hijacked vehicle was found abandoned late on Thursday between the Dadaab camp and the border, which lies about 100 km (60 miles) away.

The campaign group Human Rights Watch reported in 2009 that al Shabaab was recruiting fighters from inside Dadaab.

A Kenyan driver working for the international relief group Care is still missing after he was grabbed in September from the Dadaab camp.

The Geneva-based UNHCR said it has 200 staff in Dadaab. A further 100 humanitarian workers work alongside them, from aid agencies including MSF, it said.