LIBREVILLE (AFP) - Cameroon, Chad and Niger have launched a regional bid to combat Boko Haram Islamists, as their attacks spread beyond Nigeria and concern mounts over the Nigerians' failure to regain control.

The three neighbours have opted for a joint military response to the cross-border threat from Boko Haram fighters and have made veiled criticisms of Nigeria, whose armed forces appear no match for the Islamist group that emerged in 2009.

Brutal raids, massacres, suicide bomb attacks and kidnappings by Boko Haram have claimed at least 13,000 lives and driven an estimated 1.5 million people from their homes, mainly in arid north-east Nigeria.

Officially, all four states, whose borders converge in remote territory at Lake Chad, formed a military alliance that was due to take shape last November to battle Boko Haram.

But building a combined Lake Chad force seems to have dropped off the agenda.

Now the urgency of the situation is such that Nigeria's partners appear to have finally lost patience and decided to act.

Cameroon in particular has been critical of what it sees as the Nigerian authorities' passivity in the face of Boko Haram.

On Friday, after Chad's parliament voted to send armed forces to Cameroon and Nigeria to help fight the Islamists, Chadian army vehicles headed south out of the capital N'Djamena.

'BOKO HARAM'S DEADLIEST ACT'

The move comes after the Islamists seized Baga town on the Nigerian shore of Lake Chad early this month in an offensive that "could be Boko Haram's deadliest act", according to Daniel Eyre, Nigeria researcher for Amnesty International.

Satellite pictures released by Amnesty and Human Rights Watch last week showed widespread destruction with around 3,700 buildings in Baga and nearby Doron Baga damaged or destroyed.

Eyre said as many as 2,000 civilians may have been massacred, but Nigeria's army objected to the "sensational" claims and said that the death toll in Baga was about 150.

The Islamists detained "over 500 women and hundreds of children" in a school, one woman who escaped the area told AFP, adding that she had seen "decomposing bodies scattered all over".

US Secretary of State John Kerry on Thursday joined rights monitors in blaming Boko Haram for "a crime against humanity" on the basis of evidence from the towns in northern Nigeria.

Nigeria's army had planned to use the isolated settlement of Baga as one of its key bases to work with a regional force.

However, the other countries are opposed to any major deployment inside a bastion of Boko Haram, which is against Western education and wants to establish an Islamist caliphate.

"The most worrying situation for us today is Nigeria, it's the situation of Boko Haram," Niger's Defence Minister Karidjo Mahamadou said after the fall of Baga.

"Since November, we have no longer been at that post (Baga). We explained to the Nigerians that we could not stay since we did not wish to put the lives of our soldiers in danger," Mahamadou said.

'KICK OUT THE EVIL FORCES'

Like Niger, Cameroon was strongly opposed to the Baga deployment option and will not send any troops into Nigeria on a permanent basis, security sources said after officials said the army killed 143 Boko Haram fighters who had attacked a military base in the northern town of Kolofata.

President Paul Biya favours the exchange of intelligence reports to enable coordinated operations, but believes each nation should act on its own territory around Lake Chad, the security sources said.

Cameroon has for months complained about the Nigerian army's lack of fight and mass desertions in the face of the Islamists.

"Nigerian soldiers abandon their weapons when they desert their positions," a Cameroonian military officer said early this week.

"Those are the weapons with which we are attacked."

For Chad, its battle-hardened army will step in to help "in the courageous and determined response of (Cameroon's) armed force against the criminal and terrorist acts of Boko Haram," the government stated Wednesday, after President Idriss Deby received Cameroon's defence minister.

And on Saturday tens of thousands of Chadians marched in the capital to show support for the fight against Boko Haram chanting in French and Arabic: "Kick the forces of evil out of our territory."

Chad's "vital interests" are at stake, officials have said, but N'Djamena is still cautious. When troops from both Chad and Niger withdrew from Baga, they were "unable... to accomplish their mission with the Nigerian army," a Chadian official said, asking not to be named.

The criticism of Nigeria's battle against Boko Haram comes as the stakes are mounting ahead of a presidential election on Feb 14.

Incumbent Goodluck Jonathan, who is seeking re-election, paid a surprise visit Thursday to Boko Haram's heartland, proffering hope less than a month before polls - but whether he can back that up remains uncertain.