Kenyan and Somali forces pursued rebel al-Shabab fighters in southern Somalia on Monday, in an offensive to drive militants linked to a wave of Western kidnappings away from Kenya's border.
Warplanes launched air strikes on two al-Shabab bases over the weekend and a Somali military commander said his troops were closing in on the town of Afmadow, previously a rebel stronghold.
Under pressure to beef up security along its porous frontier after a string of attacks on Westerners by gunmen thought to be connected to the al Qaeda-linked rebels, Kenya is desperate to limit damage to its reputation as a relatively secure tourism and investment destination.
"Kenyan troops with heavily armoured vehicles have reached Qoqani village and are preparing to move on this morning," Ali Mohamud, a resident of Qoqani which lies between the border and Afmadow, 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.
The latest operation appeared to be a significant escalation in military involvement but one which also risks dragging Kenya deeper into Somalia's two-decade civil war and raises the risk of retaliatory attacks on Kenyan interests by al-Shabab.
Somali Colonel Janwaase Mahdi told Reuters his soldiers were advancing on the town of Afmadow, near an al-Shabab base hit by air strikes 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-Shabab back," Mr. Mahdi said.
Keen to avoid a spillover of violence by al Qaeda-trained foreign jihadists seeking haven in Somalia as well as al-Shabab rebels entrenched in the south, Nairobi has in the past contemplated creating a buffer zone along its border.
Kenya has already trained thousands of newly recruited Somali soldiers to man the frontier. It also provides logistical and intelligence support to Somali government troops and government-friendly militia.
"The Kenyan military strategy looks intent on supporting these military groups," said Rashid Abdi, a Nairobi-based Somalia analyst at the International Crisis Group.
"I don't think Kenya has any intention of occupying southern Somalia. For one, they have no capacity to do that," he said. "What they probably want to do is step up the support of these militia groups so that the militia groups take territory."
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 last week has threatened Kenya's lucrative tourism sector and forced the government to show it can defend its frontiers.
The militants denied they were behind the kidnappings but security sources say the British and French women are being held in al-Shabab controlled territory in central Somalia, highlighting cooperation between the militants and criminal networks such as pirates who hijack vessels in the Indian Ocean.
The rebels struck the Ugandan capital Kampala in July last year, killing more than 70 people in an attack they said avenged the deployment of Ugandan peacekeeping troops in Mogadishu.
Horn of Africa experts say al-Shabab has resisted following through on its own threats to target Kenya, which diplomats say is an important money laundering destination and, in places, fertile recruitment zone for the rebels.
"There will now be a strong argument now for 'let's punish Kenya for this'," said ICG's Abdi.
Al-Shabab'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 wants to impose a strict version of its own interpretation of sharia (Islamic law) and more hardline factions within al-Shabab want to attack Somalia's neighbours.