Maltam - The army of Chad is putting 400 military vehicles, attack helicopters, and a still unspecified number of soldiers amassed in northern Cameroon into action against Boko Haram, as part of a regional effort to defeat the notoriously violent group.

"We are going to advance towards the enemy," Chadian army colonel Djerou Ibrahim, who is leading the offensive against Boko Haram, told AFP from the strategic crossroads town of Maltam in northern Cameroon.

"Our mission is to hunt down Boko Haram, and we have all the means to do that."

But Cameroonian Communications Minister Issa Tchiroma Bakary warned the armies of the two nations still had considerable planning to complete before being able to launch offensives against Boko Haram.

"Military planners must evaluate the forces being co-ordinated and coalesced," he said. "That takes time."

Hostages

Boko Haram freed two dozen hostages after a mass abduction by its militants in Cameroon, as Chad prepared on Monday to engage in the international battle against the extremist group.

Twenty-four of the 80 people taken hostage by Boko Haram in the north of Cameroon on Sunday were released as Cameroonian armed forces pursued the Islamist extremists, according to a government source.

The Boko Haram fighters then fled back into Nigeria, with the fate of the rest of the hostages taken in the raid still unknown.

An army officer based in Cameroon's Far North region said Boko Haram had attacked two villages and kidnapped what Cameroonian state media said were 80 hostages, in one of the group's biggest abductions outside Nigeria yet.

Three people also died in the assault.

Chadian President Idriss Deby has clearly stated his determination to re-capture the strategic town of Baga in northeastern Nigeria, which Boko Haram stormed in murderous attacks in early January.

Crimes against humanity

Witnesses recounted terrifying scenes of violence - and the abduction by Boko Haram of what some said were more than 500 women and children - which French President Francois Hollande and US Secretary of State John Kerry described as "crimes against humanity".

According to Amnesty International, the attack of Baga was "the biggest and most destructive" by Boko Haram since it launched its campaign in 2009 to create a caliphate in the region, which observers say has claimed at least 13 000 lives and driven an estimated 1.5 million people from their homes.

"The attack by Boko Haram on the border town of Baga is the latest in a long series of atrocities killing hundreds and displacing thousands of innocent people," European Union foreign policy chief Federica Mogherini said in a statement on Monday.

"Increasingly frequent attacks, notably into Cameroon, demonstrate that the threat from Boko Haram has taken on a regional dimension."

Chad has thus far been spared attack by Boko Haram, but only a thin sliver of land in northern Cameroon separates the desert state from the Islamists' stronghold in Nigeria's Borno state.

Chad has also been affected by the refugee crisis sparked by Boko Haram's insurgency, and Deby has warned he will "not stand idly by" as the extremists enlarge their field of activity.

Nigeria unable

Nigeria, which has been unable to halt Boko Haram on its own, expressed its conditional support of Chadian soldiers eventually being deployed on its soil.

"All backing of our operations will be welcomed, but that must conform to operations we already have under way, given those are on Nigerian territory," said Nigerian army spokesperson Chris Olukolade.

The leaders of Ghana and Germany, who met in Berlin on Monday, supported using EU money to help fund a regional African force to battle the Islamists.

"I believe it is right to choose African troops for this task but it is our common interest that we sustainably finance such a force," Chancellor Angela Merkel told reporters.

Ghanian President John Dramani Mahama said "it would be very important if our partners could join in terms of how we finance that force".