* Gunmen take aid workers from remote Kenyan town

* Kidnappers move deeper inside Somalia

* Al Shabaab seizes one of captors' cars

(Adds details of car seizure, paragraphs 6-8)

By Mohamed Ahmed

MOGADISHU, July 18 (Reuters) - Somali gunmen kidnapped three foreign aid workers on Saturday in a raid on a Kenyan border town, then went back over the porous border into the Horn of Africa nation, rebels and residents said.

Cross-border raids are fairly common in the remote region, but usually involve cattle rustlers or gangs of robbers preying on business people in both countries. Ill-funded Kenyan security forces can do little to police the vast, impoverished area.

Somalia's militant al Shabaab group, which denied involvement, vowed to track down the captors who took the three workers from Mandera town, straddling the Kenya-Somali border.

"The authorities in Mandera (in Kenya) told us that those aid workers had been kidnapped. We're now going to run after them," said Sheikh Osman, an al Shabaab member in the neighbouring district in Somalia.

Al Shabaab said the kidnappers were moving deeper inside Somalia and had been seen at the entrance to the provincial town of Baidoa where the group had seized one of the captors' cars.

Sheikh Aden of al Shabaab said the vehicle with the hostages had escaped and was heading towards Mogadishu.

"One of the vehicles has been seized ... our men surrounded the car, which was escorting the car carrying the hostages ... we're still chasing after the hostages," he said from Balad Hawa, the town neighbouring Mandera on the Somali side.

There was no independent confirmation of that report.

No group has claimed responsibility, but al Shabaab blamed members of another Islamist rebel group, Hizbul Islam, for the attack. Hizbul Islam was not immediately available for comment.

The two groups are usually allies in their fight against the government, but differences and disputes have emerged. Somalia's varied insurgent alliances are often further complicated by clan loyalties and local disputes.

WAKE UP TO GUN SHOTS

The aid organisation has asked that its name and the nationalities of the hostages not be released. In the past, most foreigners kidnapped have been released after a ransom payment.

Residents in Mandera said an unknown number of assailants entered the town early on Saturday and seized the three.

"I got up to gun shots, and it was over in a very short time. In fact, there had been no resistance at all except from the guard whom they shot in the head and he is at the hospital now," resident Abdi Mohamed told Reuters by telephone.

Suspicion of kidnappings usually falls on clan militias or insurgents. Seizures are usually of Somalis, sometimes of foreigners and increasingly of ship crews off the coast.

Somali pirates released a German-owned ship on Saturday after receiving a $1.8 million ransom.

On Tuesday, two French security officials were kidnapped in the Somali capital Mogadishu and eventually handed over to al Qaeda-linked al Shabaab rebels.

Somalia's government and a 4,300-strong African Union force have been unable to gain control of rebel strongholds in Mogadishu and other parts of the country despite international support and training.

A two-year insurgency there has killed at least 18,000 people, according to a local rights group.

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