Kenyan security forces have begun assembling along the border with war-torn Somalia in preparation for assaults on militia forces behind several recent kidnappings of foreigners.

"Our forces are assembling at the border points where they are being briefed... as they prepare to enter Somalia," said an official at military headquarters on Sunday, asking for anonymity as he was not authorised to speak to the media.

Kenya's Internal Security Minister George Saitoti said on Saturday that troops would cross the frontier, branding Somalia's Al-Qaeda-inspired Shebab rebels "the enemy" and blaming them for the abduction of four European women.

Over the past month, a British woman and a French woman have been abducted from beach resorts in two separate incidents, dealing a major blow to Kenya's tourism industry.

On Thursday, two female Spanish aid workers were seized from the Dadaab refugee camp, the world's largest and crowded with some 450,000 mainly Somali refugees. "They (the army) have been instructed to get ready for the assignment, which will mainly include pushing the Shebab rebels far inside Somalia away from the common border," the military official added.

Kenya already backs anti-Shebab and pro-government militia groups in Somali border regions as efforts to create a buffer zone from hostile rebels. An AFP reporter close to the border witnessed large numbers of troops as well as military airplanes and helicopters overhead.

Several witnesses reported heavy troop movements in Kenya's border regions, with truckloads of soldiers heading towards the frontier.

While Kenya has blamed the abductions on the Islamist Shebab movement, experts say the kidnappings could also be the work of pirates, bandits or opportunistic criminal gangs.

Somalia has had no effective government since it plunged into repeated rounds of civil wars beginning in 1991, allowing militia armies, extremist rebels and piracy to flourish.