POTISKUM, Nigeria - A girl as young as 10 blew herself up in a busy market in northeastern Nigeria, killing herself and four others, and fueling fears Islamic extremists are using kidnapped girls as suicide bombers.

There was no immediate claim of responsibility for the attack Sunday, which also seriously wounded 46 people, but it bore the hallmarks of Nigeria's homegrown extremist group, Boko Haram.

The girl, who appeared to be no more than 10 years old, got out of a tricycle taxi in front of the busy cell phone market in Potiskum and minutes later her explosives detonated, according to Anazumi Saleh, a survivor of the attack who suffered head injuries.

Get Breaking News Delivered to Your Inbox

Authorities were not immediately able to confirm the girl's identity or her precise age from her remains.

In recent months, Boko Haram has begun using teenage girls and young women for suicide bombings in marketplaces, bus stations and other busy areas, but the girl in Sunday's attack appeared far younger. It is not clear whether the girls and women have set off the explosions themselves, or whether the detonations were controlled remotely.

Boko Haram, whose name means "Western education is sinful" in the local Hausa language, attracted international condemnation when its fighters kidnapped 276 mostly Christian schoolgirls from a boarding school in the northeastern town of Chibok in April. Dozens escaped but 219 remain missing. Boko Haram has said the girls have converted to Islam and been married off to extremist fighters.

Boko Haram's violent campaign in Nigeria killed at least 10,000 people last year, according to the Council on Foreign Affairs. At least 1.6 million people have been driven from their homes in the group's brutal five-year uprising to create an Islamic state in Nigeria, Africa's most populous nation of 160 million people divided between mainly Muslims in the north and Christians in the south.

Potiskum, the capital of Yobe state, has been the target of repeated attacks. In November, a suicide bomber disguised in a school uniform set off explosives hidden in a backpack during an assembly at a high school, killing at least 48 students and wounding 79 others.

Meanwhile, Boko Haram denied a Ministry of Defense statement that Nigeria's military had retaken the border garrison town of Baga.

It has been reported that troops from Nigeria and neighboring Chad were retaking towns and villages held for months by Boko Haram even as the extremists attack other northeastern communities. Scores of civilians have been killed in such attacks in recent days.

"Baga still is under the control of the mujahedeen and any claim by the regime that they took the city is their usual lie," said a brief message posted on the Twitter account of Al-Urwa Al-Wuthqa, a group that releases propaganda for Boko Haram, according to the SITE intelligence monitoring service.

The Associated Press was trying to verify the situation in Baga, a town on Lake Chad near the border with Cameroon where the extremists are accused of killing hundreds of people in a January attack after Nigerian troops fled.

The government hopes the military will be able to reclaim enough territory to allow presidential elections March 28, which Boko Haram is threatening to disrupt.

The vote looks to be the most closely contested ever in Nigeria, Africa's biggest oil producer. Boko Haram, which denounces democracy as a corrupt Western concept, has warned it will disrupt the elections by attacking polling stations.

The group has indicated it may soon join up with the Islamic State of Iraq and Syria (ISIS), according to a message posted Sunday on Twitter, according to SITE. Boko Haram began emulating ISIS last August, declaring it had established an Islamic caliphate in territory it controls in northeastern Nigeria.

"We give you glad tidings that the group's Shurah Council is at the stage of consulting and studying, and we will let you know soon the group's decision in respect to pledging allegiance to the caliph of the Muslims, Abu Bakr al-Baghdadi," the group said in the message, referring to the Islamic State leader.