* Victims are Spanish women working for MSF
* Al Shabaab rebels deny involvement
* Famine in Somalia this year has swollen camp's numbers (Adds name of one of hostages from Spanish media)
By Daud Yussuf
GARISSA, Kenya, Oct 14 (Reuters) - Gunmen were on the run with two Spanish aid workers kidnapped on Thursday from Kenya's Dadaab refugee camp, after the third abduction of Westerners in Kenya by attackers linked to Somalia in a month.
Kenyan and Somali security forces were hunting for the kidnappers along the border between the two countries, which has been sealed off.
Police said they suspected Somalia's al Qaeda-linked al Shabaab insurgents were behind the kidnapping of the two women who work for Medecins Sans Frontieres (MSF).
One of the two women has been named as Montserrat Serra i Ridao, a native of Gerona, northeast Spain, Spanish media reported, citing MSF and Spain foreign ministry sources. The other aid worker is from Madrid, the reports said, but provided no further details.
No one at the Foreign Ministry was immediately available for comment. MSF had declined to disclose the identity of the two women at a news conference in Barcelona earlier on Thursday evening.
A senior al Shabaab official, who did not want to be named, dismissed the claims. "We heard about the MSF abductions but we were not behind it," the official in southern Somalia told Reuters by telephone.
"Nor have they been brought into any area under our control."
Al Shabaab controls large parts of southern and central Somalia, including areas close to the border with Kenya.
The Spanish foreign ministry said in a statement that it is in contact with the families of the kidnapped aid workers and it is "doing everything it can to free the two women."
A local Kenyan intelligence source said security forces were investigating reports a pastoralist had stumbled across the women's vehicle, abandoned between Dadaab and the frontier.
Somali lawmaker Abdi Bule Hussein, from the Lower Juba border area, said he too had heard the vehicle had been ditched.
Heavy rain lashed the semi-arid region on Thursday, making driving treacherous on the sandy tracks that criss-cross the area and hampering the rescue operation.
"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 in which Somali gunmen with close ties to pirates seized Western female tourists from Kenyan beach resorts.
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 in September from the Dadaab camp.
Kenya's security forces said they did not know whether the abductors and their Spanish captives had crossed into Somalia.
"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," North Eastern Province police commander Leo Nyongesa told Reuters.
Dadaab, located about 100 km (60 miles) from the Somali border, was set up in 1991 to house Somalis fleeing violence in their country. The camp's population has swollen to more than 460,000 people this year because of famine in Somalia.
"This is disturbing and a setback," said the U.N. refugee agency's chief spokeswoman Melissa Fleming.
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 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. (Additional reporting by Nour Ali in Isiolo; Yara Bayoumy, Humphrey Malalo and Richard Lough in Nairobi, Feisal Omar and Mohamed Ahmed in Mogadishu,; Tom Miles and Stephanie Nebehay in Geneva, Cristina Fuentes-Cantillana, Catherine MacDonald and Judy MacInnes in Madrid and John Irish in Paris)