UN Secretary-General Ban Ki-moon strongly condemned on Tuesday Boko Haram attacks that deliberately targeted Christian and Muslim worshipers during the Muslim holy month of Ramadan in northeastern Nigeria.

More than 100 people died in the latest wave of Boko Haram attacks on three towns in Borno state since the end of last month, including an explosion triggered by a woman suicide bomber that killed as many as 40 people.

"I saw three truckloads of body parts taken away by the police," said Aisha Mohammed, who was in the corridor at the government headquarters where the explosion occurred on Tuesday morning.

Some bodies were charred beyond recognition, a reporter from The Associated Press who was at the morgue reported.

Governor Nasir El-Rufai urged citizens to avoid crowded public places, including mosques and churches, as the militants accelerate the pace of attacks that have killed some 300 people in a week. Boko Haram may be responding to an Islamic State group order to commit more mayhem during Ramadan.

Terrorists detonated explosives that killed and injured several people, with 25 fatalities, including a 2-year-old, El-Rufai said.

Fifty wounded victims were hospitalized, his spokesman Samuel Aruwan said.

Also on Tuesday, Boko Haram fighters crossed the border into Cameroon, and three were killed in a fierce clash with troops, Cameroon government spokesman Issa Tchiroma Bakary said.

He said a large quantity of weapons and vehicles were seized before the attackers retreated into Nigeria.

On Sunday, six people were killed in a suicide bomb attack on a church in Potiskum, a commercial town in the northeastern state of Yobe, Nigeria.

Xinhua - AP