At least 27 people, including four suspected Boko Haram fighters, have been killed in a series of attacks in Chad's capital, N'Djamena, according to a government spokesperson.

Monday's bombings outside the police headquarters and police academy were the first such attacks in the city, where security has been tightened since Chad joined the fight against Boko Haram earlier this year.

An official with N'Djamena's police force told AFP that many people were dead and wounded in the twin attacks although there was no precise casualty toll.

There was no immediate claim of responsibility for the bombings, which came as police cadets were attending a training course at the academy.

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Large numbers of Chad's security forces were seen taking up positions on the streets of N'Djamena after the attacks.

President Idriss Deby was expected to return home during the day from an African Union summit in Johannesburg, an official said.

Emergency meeting

Hassan Sylla Bakari, Chad's communications minister, said on state television that Boko Haram had made a mistake targeting Chad and its members would be located and neutralised.

Chad's government quickly convened an emergency meeting following the bombings outside the police headquarters and police academy in N'Djamena, an official said on condition of anonymity.

Chad, a former French colony, is part of a four-nation coalition also including Nigeria, Cameroon and Niger that was created to tackle the Boko Haram insurgency as the group steps up cross-border attacks.

Abubakar Shekau, Boko Haram's leader, has on several occasions threatened to attack Chad and other countries in the coalition.

France condemned the blasts, with a foreign ministry spokesperson saying France "stands alongside Chad and its partners in the fight against terrorism".

Chad also is a close ally of France in its counterterrorism Operation Barkhane in five countries in the Sahel region and the French army has set up its headquarters for the campaign in N'Djamena.

Last week, Abuja hosted a summit where Nigeria and fellow coalition members plus Benin gave their approval to an 8,700-strong regional force to replace the current four-nation grouping.

The long-awaited Multi-National Joint Task Force, which was due to have been operational in November, has its headquarters in N'Djamena, under a senior Nigerian officer.

Under attack

Chad's involvement in the fight against Boko Haram began in January when Deby sent troops to assist neighbouring Cameroon, whose far northern region was coming under attack from the rebels.

More than 70 Chadian soldiers have died in operations against the fighters, including attacks around Lake Chad where the borders of Nigeria, Chad, Cameroon and Niger meet.

Boko Haram has been waging a six-year campaign of violence in northeastern Nigeria that has left at least 15,000 people dead and increasingly spilled across borders.

Boko Haram, whose name roughly translates as "Western education is forbidden", aims to create a state ruled by Islamic law in the territories it controls and, earlier this month, declared allegiance to the Islamic State of Iraq and the Levant (ISIL) group.

Some of the 1.5 million people made homeless by the violence have fled to Chad, a poor, largely desert landlocked country.