Somali forces allied to Kenya attacked an al Shabaab base near Afmadow on Friday and said they would press towards the strategic town seen as an obstacle to seizing the key port city of Kismayu.
Kenyan troops entered Somalia nearly five weeks ago to dismantle the al Qaeda-linked rebels' network in southern Somalia and are fighting alongside Somali government troops and the Ras Kamboni militia.
After a relatively resistance advance, the forces fighting al Shabaab have since been camped near several rebel strongholds, but have yet to have a major showdown with the insurgents and heavy rain is hampering progress.
A spokesman for Ras Kamboni told Reuters they and Somali troops (TFG) attacked the base, killed four rebel fighters in an hour-long clash in muddy terrain and captured two military vehicles.
Kenyan military spokesman Emmanuel Chirchir confirmed the attack, although no Kenyan soldiers were involved.
Yes, there was an attack. The TFG attacked an al Shabaab base about 7 kilometres (4 miles) from Afmadow, he told Reuters. I hear four Shabaab were killed and one captured.
Sheikh Abdiasis Abu Musab, the spokesman for al Shabaab fighters, confirmed they had been attacked on Friday but said they had driven the forces back and killed three soldiers.
Kenya sent troops into its anarchic neighbour to chase the rebels after a series of kidnappings and incursions on its soil that threatened the tourism industry and national security.
Al Shabaab has denied being behind the seizures of Western aid workers from the Dadaab refugee camp and tourists from northern Kenyan beach resorts and says its neighbour used them as a pretext to launch the incursion.
Kenyan government officials have said the kidnappings were really the last straw, as al Shabaab had long been a threat to the interests of east Africa's biggest economy, especially major infrastructure projects.
Kenya is hoping to build a world-class port in the Lamu region not far from the Somali border
The $23 billion project, which includes a $5.3 billion port, will link Lamu to Ethiopia and newly independent oil-producing South Sudan, bringing investment and much-needed jobs to the region. A railway, pipeline, highway, airport and refinery would follow.
Since Kenya moved into Somalia there have been a string of deadly attacks in the capital Nairobi and northern areas not far from the border between the two neighbours.