N'Djamena (AFP) - Chad on Wednesday banned the full-face Muslim veil and ordered security forces to seize burqas from markets and burn them after 33 people were killed in suicide bombings blamed on Nigerian Islamist group Boko Haram this week.

"Wearing the burqa must stop immediately from today, not only in public places and schools but throughout the whole of the country," Prime Minister Kalzeube Pahimi Deubet said in a speech to religious leaders the day before the start of the holy Muslim festival of Ramadan.

Any type of clothing that leaves only the eyes visible is a form of "camouflage" and is now banned, he added, asking the religious leaders to spread the message in their mosques, churches and holy places.

Prime Minister Deubet said instructions had been given to security forces to "go into the markets and to seize all the burqas on sale and burn them".

Anyone found wearing a burqa would be "arrested, tried and sentenced in summary proceedings", he added.

Monday's bombings, the first such attacks in the capital of Muslim-majority Chad, have been blamed on Boko Haram jihadists who have previously carried out bloody assaults on villages along the border with Nigeria.

The Islamist militants have used female suicide bombers to launch attacks in the past by hiding explosive devices under their clothes.

Chad's government on Tuesday declared three days of national mourning for the 33 people killed and more than 100 others wounded in the blasts.

President Idriss Deby said he was "not surprised" the country has been targetted because of the leading role its army is playing in a regional offensive against Boko Haram fighters operating out of northeastern Nigeria.

- 'Don't drop your guard' -

"I have continually told the government to not drop its guard," he said, urging the international community to back the states in their struggle.

Nigeria, Chad, Niger, Benin and Cameroon agreed last week to set up a regional task force of 8,700 soldiers, police officers and civilians, based in N'Djamena.

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Boko Haram's leader Abubakar Shekau had threatened several times to attack Chad and other countries that joined forces against the militants, whose bloody six-year insurgency is increasingly spilling across Nigeria's borders.

Although Boko Haram has yet to claim responsibility for the bombings, France, which relies heavily on N'Djamena in the fight against jihadist groups in the Sahel region, accused the militants of being behind the attack.

UN Secretary General Ban Ki-moon also condemned Monday's attacks and praised Chad "for its courageous role in the fight against Boko Haram".

Security was stepped up in N'Djamena Tuesday, with scores of police and soldiers patrolling the streets and stopping cars for security checks.

Vehicles with tinted windows had been barred from the streets, and the area around the presidential palace and the police headquarters -- which was one of the bombers' targets, along with a police academy -- had been sealed off.

The burqa ban was ordered by a crisis committee formed on Tuesday after the president returned from an African Union summit in South Africa. Prosecutors also arrested several people on the same day.

"Strict instructions have been given to the government and security services... additional measures were (also) taken by the head of state," said Deubet.