A large explosion was heard near a Somali rebel stronghold about 30 km (19 miles) from the capital Mogadishu on Sunday, residents and a senior rebel official said.

They said the blast hit near Afgoye, one of 10 Somali towns the Kenyan military said risked air strikes due to the presence of al Shabaab rebel bases.

I've never heard such an explosion in my life, Afgoye resident Mohamud Abdi Ali told Reuters. It didn't explode in the town, but I think it exploded near al Shabaab's military camps.

A farmer, who lives near the rebel camps at Afgoye and gave his name as Abdi, said a missile struck buildings outside the town used by al Shabaab fighters for meetings.

Afgoye is a strategic junction on the road leading from the capital to the south of the Horn of Africa nation.

I heard a huge explosion outside Afgoye myself, but I do not know what it is, a senior al Shabaab official, who declined to be named, told Reuters.

The United States has used drones in the past to target top al Shabaab officials.

Kenya's military spokesman was not available for comment.

Kenya, the region's biggest economy, sent troops into Somalia five weeks ago to rout the insurgents it blames for kidnappings of Western aid workers and tourists on Kenyan soil, and frequent cross-border incursions.

After a fairly smooth advance, Kenya's forces fighting al Shabaab have since been camped near several rebel strongholds, but have yet to have a major showdown with the insurgents, nor seize any significant strategic bases.

CLASH

Other residents living between Mogadishu and Afgoye said they saw a bright light streaking overhead in the night sky.

After some seconds we heard a big explosion that shook all our houses. It either exploded inside Afgoyo or nearby, said Ashao Ahmed, who lives outside Mogadishu.

Four Kenyan soldiers were wounded in a firefight with Somalia's al Shabaab rebels overnight, the latest in a series of recent clashes near the border between the neighbours.

The clash overnight on Saturday was about 20 km inside Somalia from the border town of El Wak.

Kenyan military spokesman Emmanuel Chirchir said their forces and Somali government troops bumped into an al Shabaab camp while on patrol late at night.

He said in a statement that four Kenyan soldiers had been wounded, one critically, and nine al Shabaab fighters killed. He also said seven people had been arrested in a raid at El Wak and a number of weapons recovered.

The Kenyan army said on Saturday its progress had been hampered by heavy rain in an area with few Tarmac roads, and because its troops were rooting out al Shabaab sympathisers in areas it now controls while feeding the famine-hit population.

It takes time. When we are done with that we will be able to move forward, said Colonel Cyrus Oguna.

He also said al Shabaab fighters had shifted their tactics to guerrilla-style warfare, using roadside bombs, grenade attacks and hit-and-run raids.

Oguna said on Saturday that Kenya had lost five soldiers since the start of the campaign and a couple had been wounded. He said several hundred al Shabaab fighters had been killed.

(Additional reporting by Noor Ali in Garissa and David Clarke in Nairobi; Writing by David Clarke; Editing by Janet Lawrence)