Gunmen kidnapped two Spanish women working for Medecins Sans Frontieres (MSF) at Kenya's Dadaab refugee camp on Thursday, the third abduction of Westerners in Kenya by attackers linked to Somalia in a month.
Kenyan police said they suspected Somalia's al Qaeda-linked al Shabaab insurgents were behind the kidnapping and that security forces had chased the abductors towards the border between the two countries, which has been sealed off.
No group has claimed responsibility for the attack.
Two female aid workers working for MSF were ... kidnapped by suspected al Shabaab militants in Dadaab refugee camp, North Eastern Province police commander Leo Nyongesa told Reuters.
We've mobilised all the officers and alerted those at the border to ensure that no vehicle exits the country to Somalia. The whole border area is now sealed, he said.
MSF said it had been unable to contact the two hostages and said it would not reveal their names until their families had been informed.
We strongly condemn this attack, José Antonio Bastos, the president of MSF-Spain, said in a statement. MSF is in contact with all the relevant authorities and is doing all it can to ensure the swift and safe return of our colleagues.
A spokesman at the Spanish Foreign Ministry confirmed the missing women were Spanish.
Aid workers have been targeted for abductions on numerous occasions in Somalia, where kidnappings can be a lucrative business, but attacks in Kenya had been relatively rare until a recent spate of incidents.
In 2009, three foreign aid workers working for the French charity Action Contre la Faim (ACF) were nabbed by Somali gunmen from the Kenyan border town of Mandera. Two Western nuns were kidnapped in 2008.
Thursday's incident took place within weeks of two separate incidents that saw Somali gunmen with close ties to pirates seize Western female tourists from beach resorts in northern Kenya.
Analysts and diplomats in the region had warned that pirates were likely to turn to softer targets, such as tourists in Kenya, in response to much more robust defence of merchant vessels by private security guards.
Security experts fear Islamist militants fighting to topple the Western-backed Somali government could increasingly conduct copycat attacks inside Kenya, the region's biggest economy.
A Kenyan driver working for the international relief group Care is still missing after he was grabbed at gunpoint in September from the Dadaab camp.
A senior Kenyan military officer acknowledged the security forces did not know whether the abductors and their Spanish captives had crossed in Somalia.
We have four choppers flying in our airspace along the border between Dif and Liboi, said the army officer who declined to be named.
Dadaab, located about 100 km for the Somali border, was set up in 1991 to house Somalis fleeing violence in their country. It has since grown to become the world's biggest refugee camp with more than 400,000 residents.
MSF said a Kenyan driver was wounded in Thursday's attack.
The kidnapping will put further pressure on the Kenyan government to beef up defences along its porous frontier and risks further hurting the tourism sector, one of the country's top foreign currency earners.
Britain has already issued a travel advisory warning against all but essential travel within 150 km of the Somali border, which includes the popular Lamu archipelago where a French woman and a British woman were seized in past weeks.