Somali Islamist militants mounted weapons on roofs, dug trenches and armed students in the port of Kismayu, warning the Kenyan invasion would lead to cataclysmic consequences.

Kenya's army has warned Somali civilians to stay away from al Shabaab militant bases in 10 towns to avoid being hurt in imminent strikes. But nearly three weeks into a cross-border operation, however, Kenyan and Somalian government troops have been bogged down by heavy rains and thick mud.

They have put their weapons over us. Every high house in the city is a defence for al Shabaab, said Fatuma Ali, who lives next to the rebel base in Kismayu.

Since Kenya mentioned the 10 towns, al Shabaab have been readying all their weapons and small arms.

Angered by a wave of kidnappings and attacks on its soil, Kenya has been dragged into the conflict in neighbouring Somalia, which has had no effective government for two decades.

Al Shabaab has denied responsibility for the kidnappings, saying Nairobi is using them as a pretext for its operation.

Kenya issued its warning of imminent attacks after it said it had received intelligence that consignments of weapons had reached al Qaeda-linked militants in the town of Baidoa.

Kenya's army spokesman said on Thursday any aircraft landing in the rebel bastion would be considered a threat.

All aircraft are hereby warned not to land in Baidoa. Anyone violating this will be doing so at their peril, Emmanuel Chirchir said in a statement.

Kenya's navy intercepted a rebel skiff transporting fuel on Wednesday and killed 18 al Shabaab combatants, he said.

Al Shabaab said Kenya was using the Baidoa arms cache discovery as an excuse to bomb the rebel towns.

It is apparent that the operation is not simply an attempt to defend Kenya's territorial boundary as they claimed but rather a clever camouflage for the full-scale invasion of Somalia, the group said in a statement.

(Al Shabaab) hereby emphasises once more that the continued Kenyan invasion and the callous disregard for civilian lives will have some cataclysmic consequences.

MILITANTS MINGLE WITH CIVILIANS

Al Shabaab was arming the populace, residents said.

They gave arms to people and they're telling them to stay and defend the country from foreigners, said Kismayu resident Amina Mahmoud. They said yesterday evening: 'Every one of you who dies here is a mujahid and will enter paradise'.

Many Somalis were trying to flee towns, only to be stopped by militants who want them to stay and fight.

They've refused to let us out, and we don't have any money to leave. Some people are trying to flee but the heavy rain is not giving them a chance, said Ali.

In other towns, residents said the rebels had quit their bases and were mixing with the population, making them more difficult to pinpoint as targets.

Al Shabaab fighters are all over the town but they're no longer seen in groups. We see only four to five men in bases that were formerly crowded, Halima Aden told Reuters from Baidoa. Residents feared the strikes would hit them too.

I am sure Kenyan jets will fail to know who to bomb or not - the fighters have scattered, Ahmed Nour in Baidoa said.

On Wednesday residents in the strategic town of Afmadow said al Shabaab had also ordered them to stay at home.

Afmadow is a strategic transit point for contraband smuggled through Kismayu port and is seen as a likely flashpoint for a confrontation between Kenyan forces and al Shabaab.

Kenyan and Somali government troops, in addition to militia nominally allied to the Somalia's Western-backed government, have set up forward positions close to Afmadow.

Torrential rains have hampered the operations of both sides and forced the militants to resort to ferrying arms around the region by donkey, Kenya's military said.

Any large concentration and movement of loaded donkeys will be considered as al Shabaab activity, army spokesman Chirchir said.

He urged Kenyan donkey traders along the frontier not to sell their beasts across the border, warning they might undermine the military operation to crush the insurgents.

Kenya has east Africa's biggest economy and its troops are among the best in the region. But some analysts say it lacks the muscle to deal a mortal blow to al Shabaab, whose aim is to impose strict sharia Islamic law across Somalia.