Several Westerners have been seized in Kenya by suspected Somali militants and taken into Somalia. Two Spanish aid workers were abducted from Kenya's sprawling Dadaab refugee camp on Thursday.

A British woman and a French woman have been kidnapped from remote beach resorts over the past month, dealing a major blow to Kenya's tourism industry.

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.

Kenya, 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.

However, the militants have denied they are behind the kidnappings.

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