NIAMEY, Niger -- The Nigeria-based Islamic extremist group Boko Haram escalated its attacks in neighbouring countries Monday, as a car bomb exploded in one Niger town repeatedly targeted by the militants and residents said other fighters in Cameroon had abducted 20 people aboard a public bus.

A huge explosion rang out in the Niger town of Diffa about 3 p.m. Monday, according to Hassan Maina, who said casualties were seen being taken to a hospital. There was not immediately a toll for the attack, which was near a customs office.

Boko Haram is well known for car bombings and suicide bombings within Nigeria during its five-year insurgency, but the group had not carried out such assaults within neighbouring countries.

Monday's bombing comes as Niger's lawmakers are due to vote on a plan to send troops to help Nigeria rout the terror group blamed for 10,000 deaths over the past year. Niger, Chad, Cameroon and Benin have pledged to help Nigeria create a force with as many as 8,750 troops to fight Boko Haram.

As a result, Boko Haram has vowed to attack those countries promising to aid Nigeria.

In northern Cameroon, Boko Haram fighters seized a bus with 20 people aboard in Koza late Sunday and then drove it back toward the Nigerian border, some 11 miles (18 kilometres) away, resident Bouba Kaina told The Associated Press by telephone.

Early Monday, another Cameroonian town, Kolofata, was attacked by extremists who looted food and livestock. The town had recently been retaken by Chadian troops who have been helping Cameroon fight Boko Haram.

Diffa, the Niger town where the bombing went off Monday, already had seen three previous attacks since Friday by the group including an overnight gunbattle that lasted until Monday morning.

Boko Haram has scoffed at the regional effort, most recently in a video posted on YouTube by the group's leader, Abubakar Shekau.

"You are sending 7,000 of your soldiers. Why don't you send 7 million? The 7,000 is little and we can kill them step by step ... Your soldiers are infidels and God's soldiers are victorious," he said.

Shekau also urged the people of Chad and Cameroon to renounce democracy to be true Muslims. He ridiculed the force planned by "you tyrants of Africa" and told Chad's President Idriss Deby that he will "burn in hellfire."