Nigeria's former president Olesugun Obasanjo said on Tuesday that the root causes of the Islamist insurgency by Boko Haram need to be understood first before the problem can be resolved.

"We have to identify our problem," Obasanjo said, speaking on the sidelines of a conference on a European Union project against drug trafficking.

Nigerian authorities said Friday they had freed three women and six children abducted by Islamist extremist group Boko Haram as part of an ongoing military operation targeting the extremists.

The group was abducted on May 7 during an attack on the town of Bama in the country's northeast, according to Defense Spokesman Brigadier General Chris Olukolade. Two children and one woman remain missing, he said.

Nigerian authorities have detained children in connection with the Islamist insurgency they are battling, but the government plans to release them under a peace gesture announced this week, a statement said Thursday.

The statement from an adviser to President Goodluck Jonathan comes after widespread allegations of Nigerian authorities carrying out indiscriminate arrests, unlawful detentions and extra-judicial executions in the fight against Islamist extremists.

Nigeria said Tuesday it would release several suspects held for "terrorist activities," including all women in custody, in what it called a peace gesture to the Islamists it is battling in the north.

The announcement came as Nigeria pressed ahead with a nearly week-old offensive against Islamist insurgents in the northeast of Africa's most populous nation, while the Red Cross said at least 2,400 people had fled violence in the region.

Nigeria on Tuesday relaxed the curfew in Maiduguri, stronghold of the Boko Haram Islamist militant sect, three days after it was imposed, an Agence France Presse journalist in the city said.

City residents were able to go outside their homes during the day after the army issued a statement late Monday easing the curfew from 7 am (0600 GMT) until 5 pm (1600 GMT).

Nigeria's military said Monday it had re-established control in five remote areas of the northeast where Islamist insurgents had seized territory, as it pressed on with a sweeping offensive against Boko Haram militants.

Several thousand troops were last week deployed across three states in the volatile region, and fighter jets have pounded Boko Haram camps as the military tries to rid the country of "terrorist activities."

A key city in northeast Nigeria was on lockdown Sunday as the military enforced a 24-hour curfew and blocked supply routes in its sweeping campaign against Islamist insurgents.

The operation against Boko Haram, the group that wants an Islamic state in northern Nigeria, is aimed at retaking territory seized by the militants and ridding the country of "terrorist activities," the military has said.

Residents of an insurgent stronghold in northeast Nigeria fled their homes Saturday as military fighter jets and helicopters carried out heavy air strikes on Boko Haram Islamist camps.

Meanwhile, the military imposed later on Saturday a round-the-clock curfew in parts of the northeastern city of Maiduguri, the home base of Boko Haram.

A Nigerian court on Friday sentenced two officials from a pharmaceutical company to seven years in prison over the sale of an adulterated teething drug which killed 84 babies in 2008.

Children between two months and seven years-old, died from renal failure after taking the painkiller which was found to contain high levels of diethylene glycol, a poisonous solvent mostly used in brake fluid and as an engine coolant.

Nigeria's Boko Haram Islamists could face charges of crimes against humanity, the U.N.'s human rights office warned Friday, also urging the government to ensure civilians are not swept up in an army counter-offensive.

"Members of Boko Haram and other groups and entities, if judged to have committed widespread or systematic attacks against a civilian population -- including on grounds such as religion or ethnicity -- could be found guilty of crimes against humanity," said Rupert Colville, spokesman for the U.N. High Commissioner for Human Rights.