MOMBASA/NAIROBI (Reuters) - Kenya must win a battle against militant ideology that is seeping in from its northern neighbour and spreading to Muslim youths at home if it is to stop Somali Islamists extending their reach in east Africa.
After two attacks in 10 days by Somalia's al Shabaab group that killed more than 60 people, President Uhuru Kenyatta vowed to step up his "war on terror" to halt raids across the porous border and stop any dream of making an Islamic Caliphate.
In Somalia, he can point to military gains where Kenyan and other African troops have retaken territory from al Shabaab, but he faces a more stubborn enemy on home soil where security forces are trying to drive out militancy from mosques.
"The only language these kafirs (non-Muslims) can understand is the bullet from the AK-47 rifle," a Kenyan preacher told worshippers at Mombasa's Mina mosque last month before police shut it down - with three others - detaining about 100 youths.
Such tough tactics may temporarily silence the radical voices but it also fuels anger that helps militants find new recruits and deepens the homegrown threat, Muslim activists say.
This is where al Shabaab may have a trump card. While it has been driven out of major Somali strongholds, a military offensive in Somalia has not stopped the group spreading its ideology and finding enough loyal foot soldiers for attacks that need little more than dedication to the cause and a few rifles.
Cohorts of frustrated and often jobless Muslim youths in the sweltering port city of Mombasa and along the coast, where most Kenyan Muslims live, offer fertile ground for the Islamists.
"The government needs to sit down and understand these people," said Hussein Khalid of Haki Africa, a group that works to promote dialogue with Muslim communities. "It always wants to use force and this merely pushes people away."
Alarmed by sermons of firebrand imams and radicalised youths emerging from mosques in Mombasa, police have in the past month shut four places of worship and made mass arrests. This has been done before but not on such scale.
"We want to arrest the situation from spilling over to other mosques," said Mombasa County police commander Robert Kitur.
Al Shabaab said it launched recent raids partly to punish Kenya for such acts of "aggression" and promised to continue.
Losing territory in Somalia has not prevented al Shabaab exploiting its guerrilla skills. It may even free up resources.
"Now they are not ruling such large chunks of Somalia, they no longer have to pay for a shadow government," said a diplomat. "You can mount an insurgency for really not very much money.”
Deepening worries for the authorities is a video posted on YouTube this week with an address by Ahmad Iman Ali, a Kenyan believed to head al Hijra, the local branch of al Shabaab. He promised to target "non-believers" to avenge the suffering of Muslims in Mombasa, where Ali studied under a radical preacher.
"Wait and see, it is just a matter of time," Ali said as a group of men toting AK-47 rifles shouted: "Bullets and daggers have created warriors. Let us spread this to all mosques."
In 2007, Ali and his young supporters took over a Nairobi mosque, creating a hotbed of militancy. Radicals have done the same in Mombasa, defying repeated police crackdowns.
"It has never worked anywhere in the world," Ali Hassan Joho, Mombasa County Governor, told Reuters of his concerns about police strategy. "This is an ideological battle, therefore you cannot win it in any other way besides engagement."
Such worries are not yet turning into action. Haki Africa's Khalid said meetings between officials and the Muslim community had thrashed out strategy but there had been no "concrete meetings to confirm or at least agree on how to roll that out."
Rattled by border raids in the north and other al Shabaab attacks this year along the coast, Kenyatta urged the Muslim community to help root out Kenya's militant "collaborators".
But many moderate Muslim preachers have been cowed. In November, Sheikh Salim Bakari Mwarangi, who supported moves to stamp out radicalism, was shot dead by unknown assailants. Another moderate leader was killed in June.
"There is a very big sense of fear and people think it's not worth challenging these (radical) people," said Mudhar Khitamy, coast chairman of the Supreme Council of Kenyan Muslims.
When Mombasa's Mina mosque was shuttered, about a dozen young men chanting "Allahu akbar", or "God is greatest", marched through the street and hacked to death a Christian shopkeeper.
Kenya needs rethink strategy, said a Western diplomat: "They're not reaching the population they are trying to reach."
Additional reporting by Joseph Akwiri in Mombasa; editing by Anna Willard