A Kenyan police jeep escorting U.N. vehicles struck a landmine in the country's Dadaab refugee complex near the border with Somalia Saturday, police said, underscoring the mounting threats facing aid groups and refugees in the camp.

The explosive device failed to detonate. The incident took place close to where two Spanish women working for the medical charity Medecins Sans Frontieres (MSF) were kidnapped last month.

The police land-cruiser was escorting the morning U.N. convoy to Hagadera camp, District Commissioner Ndambuki Muthike said, referring to one of the three camps at the refugee site.

It ran over what was discovered to be a mine that had been partly buried in the sand and covered with a cooking pot.

It was not immediately clear who planted the device, which was set just a few hundred metres from a police post. Banditry is common in the area but landmines are rare.

The abduction of the Spanish MSF workers, who were taken into Somalia, was the third in a spate of attacks on Westerners in Kenya. The kidnapping helped spur Kenya into sending troops into its lawless neighbour in a bid to crush al Qaeda-linked rebels.

The al Shabaab militants have vowed to bring the flames of war across the frontier in retaliation. Security experts have also voiced concerns the rebels would increasingly turn to softer targets, such as tourists and aid workers.

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.

Separately, residents in the Kenyan border town of Hulugho said a gang of suspected Somali gunmen riding donkeys attacked the frontier post late Friday. The raiders later crossed back into Somalia.

Kenya, East Africa's largest economy, has long looked warily at its chaotic neighbour. Somali militants frequently launch cross-border assaults on border towns and Kenyan security forces.

(Writing by Richard Lough; Editing by Rosalind Russell)

(This story corrects the name of border town to Hulugho in paragraph nine from the previous story)