MAIDUGURI, Nigeria: Fighters linked to the radical Islamic terrorist network Boko Haram attacked a military base in Nigeria's north on Sunday in an assault that left at least 20 people dead, as the network's leader denied any peace talks with Nigeria's government.

The attack in the village of Monguno, some 200 kilometers (125 miles) from the city of Maiduguri, punctuated the video statement by Abubakar Shekau that said Nigeria will remain under attack by the group until the multiethnic nation is ruled under Islamic law. Shekau also threatened the man who in recent months claimed to be a leader of Boko Haram and said that the group wanted to agree to a cease-fire with Nigeria's security forces.

The attack Sunday, coupled with the recent kidnappings of foreigners claimed by Boko Haram and its affiliates, offered fresh doubts about the ability of Nigeria's weak central government to stop the bloodshed, despite its deployment of more security forces in the region.

"Whoever kills any of our members should await a grave retaliation from us," Shekau says in the video in the Hausa language of Nigeria's predominantly Muslim north. "We will continue waging war against them until we succeeded in establishing an Islamic state in Nigeria."

The attack on Monguno saw fighters arrive in SUVs and kill a local village leader, witnesses who spoke to The Associated Press said. The fighters later attacked a barracks at Monguno with gunfire and explosives, witnesses said.

In a statement, military spokesman Lt. Col. Sagir Musa said that 20 "Boko Haram terrorists" were killed, without acknowledging that at least one civilian had been killed. Musa also did not say if any soldiers had been wounded or killed in the attack. Nigeria's military routinely downplays civilian and soldier casualty figures.

Another security official, who requested anonymity as he could not speak to journalists, confirmed the attack occurred, but acknowledged details remained sketchy about the incident. An AP journalist could not immediately reach the village Sunday.

The attack Sunday comes after the release of the new Shekau video. A journalist in northeast Nigeria received the video Friday from men he said he didn't know. The journalist began sharing the video with colleagues late Saturday. While the AP could not immediately independently verify the authenticity of the video Sunday, the man on the video looked like Shekau and spoke like the Boko Haram's leader.

The video carried no date, but Shekau directly referenced the activities and claims of a man who has identified himself as Sheikh Mohammed Abdulaziz, a self-proclaimed second-in-command in Boko Haram. In November, a man with a similar voice as Abdulaziz told journalists in a telephone conference call that Boko Haram was willing to enter into peace talks if they were held in Saudi Arabia and involved former military ruler Muhammadu Buhari. However, Buhari refused to take part and no such talks took place as attacks continued.

In January, Abdulaziz told journalists in Maiduguri that a cease-fire would soon emerge that never did.

In the video, Shekau denies knowing Abdulaziz. In the past, Nigeria security forces have used so-called Boko Haram members in sting operations and to sow discord in the group.

"I swear by Allah that Abdulaziz or whatever he is calls himself did not get any authority from me to represent me in any capacity. I do not know him," Shekau says. "And if we per adventure encounter Abdulaziz and his group, I swear by Allah we are going to mete them with the grave judgment that Allah has prescribed for their likes in the holy book."

In the video, Shekau also says the group has had difficulty putting its messages online and blamed government interference for having to now rely on couriers to reach the public. The last Shekau video seen was posted to the Internet in late November.

Boko Haram, whose name means "Western education is sacrilege," has conducted a guerrilla campaign of bombings and shootings across Nigeria's north over the last two years. Boko Haram is blamed for at least 792 killings last year alone, according to an AP count. The group's command-and-control structure remains unclear, though it appears to have sparked several splinter groups.

A group of men claiming to belong to Boko Haram claimed responsibility for the kidnapping of seven French tourists from northern Cameroon late February - a first for the group. Meanwhile, a Boko Haram splinter group known as Ansaru has claimed the recent kidnappings in north Nigeria of a British citizen, a Greek, an Italian, three Lebanese and one Filipino, all employees of a Lebanese construction company called Setraco.

Despite the deployment of more soldiers and police to northern Nigeria, the nation's weak central government has been unable to stop the killings. Meanwhile, human rights groups and local citizens blame both Boko Haram and security forces for committing violent atrocities against the local civilian population, fueling rage in the region.