Kenyan and Somali forces were hunting down rebel al Shabaab fighters in southern Somalia on Monday, in an offensive to drive the militants, linked to a wave of kidnappings, away from the countries' shared border.

Warplanes launched air strikes on two al Shabaab bases over the weekend and a Somali military commander said his troops were closing in on the town of Afmadow, previously a rebel stronghold.

Kenya's government has come under intense pressure to beef up defences of its borders and inshore waters after gunmen, thought to be allied to the al Qaeda-linked rebels, seized four foreigners in a string of attacks.

"Kenyan troops with heavily armoured vehicles have reached Qoqani village and are preparing to move on this morning," Qoqani resident Ali Mohamud told Reuters by telephone. "Somali forces passed by here yesterday too," he said.

East Africa's biggest economy has long looked nervously at its anarchic neighbour and its troops have made brief incursions into Somali territory in the past, but the latest operation appeared to be a significant escalation in military involvement.

The kidnapping of a British and a French woman from the north Kenyan coast in two separate incidents and the abduction of two Spanish aid workers from a refugee camp has threatened Kenya's lucrative tourism sector and forced the government to show it can defend its frontiers.

Security sources say the British and French women are now being held in al Shabaab controlled territory in central Somalia. The militants have denied they are behind the kidnappings.

The military intervention risks dragging Kenya deeper into Somalia's two-decade civil war and raises the risk of retaliatory attacks on Kenyan interests by al Shabaab.

Somali Colonel Janwaase Mahdi told Reuters his soldiers were advancing on the town of Afmadow, near an al Shabaab base hit by airstrikes on Sunday. A Somali military commander said the rebels later fled the area. "We are heading first to Afmadow and then Kulbiyow and Badhaadhe district. We are pushing al Shabaab back," Mahdi said.

Al Shabaab's campaign to topple a Somali government it sees as a stooge of the West has killed tens of thousands of people since early 2007. The group is fighting to impose a strict version of Islamic sharia law on the nation and more hard-line factions want to attack Somalia's neighbours.