Dozens of Chadian tanks headed out of the capital Friday south towards Cameroon to help fight Nigeria's dreaded Boko Haram insurgents.

The convoy, seen by an AFP journalist, roared out of the city after Chad's parliament voted to send armed forces to Cameroon and Nigeria to fight against the Islamists.

Cameroon's President Paul Biya had announced Thursday that Chad President Idriss Deby had agreed to send "a substantial contingent" of troops to help Cameroonian armed forces, who have faced repeated attacks from Boko Haram.

A source close to the army said the force had begun preparing for departure on Thursday.

Earlier on Friday, Chad's parliament in N'Djamena voted 150 to 0 to send an unspecified number of "Chadian armed troops and security forces to assist Cameroonian and Nigerian soldiers waging war against the terrorists in Cameroon and Nigeria".

Boko Haram is fighting to create a hardline Islamic state in northeast Nigeria along the border with Chad, Cameroon and Niger. But it has recently expanded its terrifying field of activity to neighbouring countries.

Cameroon has been critical of the passivity of the Nigerian authorities and of muted international reaction in the face of Boko Haram aggression.

Since Boko Haram's insurgency began, around 135,000 people have fled the restive northeast of Nigeria, and at least 850,000 have been displaced inside the region.

So far Chad has been spared, but its border is not far from the Islamists' headquarters in the Nigerian state of Borno.

The entry of Chadian forces in the increasingly regional fight against Boko Haram may prove valuable in halting the extremists' series of ruthless offensives.

- An enormously horrendous slaughter -

After describing Boko Haram as a direct threat to the nation's "vital interests", Chadian leaders moved quickly to deploy units of the country's powerful armed forces to resume its position as one of the region's most hostile anti-jihad forces.

In 2013 Chadian troops became formidable allies of French forces battling jihadist who'd taken control of northern Mali, and played an important role in routing those extremists out into the remote areas of the Sahel.

As they did so, Chadian soldiers gained a reputation for ruthless efficiency in pursuing and liquidating retreating Islamist fighters, and were credited with killing some of the most wanted radical leaders.

One of Chad's main objectives in joining its neighbours to take on Boko Haram is re-taking the northeast Nigerian town of Baga, which the extremist group stormed in a stunning show of force January 7, provoking the flight of 5,000 people.

Over 3,500 of buildings in the strategically-situated Baga and outlying areas were thought to have been raised, perhaps 2,000 people massacred, and hundreds of people taken captive in Boko Haram's offensive.

"Boko Haram kidnapped at least 300 women and held us in a school in Baga," a non-identified woman quoted in a statement released Thursday by Amnesty International said.

"They freed the older women, the mothers, and most of the children after four days, but they are still holding the younger women," she said.

On Thursday US Secretary of State John Kerry deplored Boko Haram's activity as a "crime against humanity, nothing less," and suggested the torment of Baga was a particularly stark reminder of the threat the group poses to the region and world.

"It's an enormously horrendous slaughter of innocent people, and Boko Haram continues to present a serious threat not just in Nigeria and the region but to all of our values and all of our sense of responsibility regarding terrorism," he said.