Nigeria's Boko Haram Islamists have ruled out talks with the government and threatened new attacks in Africa's most populous country, rocked by an insurgency that has killed more than 200 already this year.

Residents in the northern city of Kaduna said the group, which has frequently targeted the police, may have struck again late Saturday when gunmen on motorcycles shot dead a policeman.

Police confirmed the killing early Sunday, but said the attackers were trying to steal the officer's motorcycle.

Kaduna resident Rabiu Tukur disputed that account.

"They did not make any attempt to take his motorcycle which raised the fear that the attackers could be members of Boko Haram," he said.

The group's deadliest ever strike came on 20 January, north of Kaduna, in Nigeria's second city of Kano, where a coordinated set of gun and bomb attacks killed at least 185 people, highlighting the Islamists' renewed strength.

Boko Haram has already claimed violence that has killed more than 200 people this year in Nigeria, Africa's top oil producer that is divided between a mainly Muslim and mainly Christian south.

President Goodluck Jonathan has been intensely criticised for his apparent inability to stem the violence and in a media interview this week urged the Islamists to state their demands and enter into dialogue.

But Jonathan's call for talks was "not sincere", purported Boko Haram spokesman Abul Qaqa told journalists by telephone in the northeastern city of Maiduguri, regarded as the group's stronghold.

"We don't think dialogue is possible under the current situation," he added.

If captured members of the group were not released, the group "will launch attacks in Sokoto similar to the big Kano attacks," Qaqa warned. Sokoto is a northwestern city which is the capital of a state of the same name.

In a recording posted recently on the Internet, the purported head of Boko Haram, Abubakar Muhammad Shekau, said he had ordered the Kano attacks because the army was torturing the group's members.

"I ordered it and I will give that order again and again," he said in the recording.

Qaqa said the government had also arrested Boko Haram loyalists in Sokoto. "They should expect imminent attacks on Sokoto if they don't release our members," he said, and warned that these illegal detentions had further undermined the possibility of dialogue.

"The same day Goodluck Jonathan came out and said we should come out for dialogue security agents in Sokoto arrested a large number of our members in raids on their homes," Qaqa said.

The Boko Haram accusations came after Nigeria's army said it killed 11 of the group's members during a Saturday operation in Maiduguri.

But Qaqa rejected the army account, saying its unarmed members "were individually picked in their homes ... and shot dead.

The Boko Haram threat has intensified in recent weeks and the Nigerian security forces have struggled to respond, with the group seemingly able to strike at will.

But as the shadowy Islamist sect steps up its violence, its specific aims and character remain largely unclear.

It has previously said that it wants to create an Islamic state in Nigeria's deeply impoverished mainly Muslim north, and has charged the government with harassing Muslims and raiding Islamic schools.

Top Nigerian politicians have denied that the Boko Haram insurgency is being fuelled by religious tensions, linking the group to like-minded external Islamist groups such as Al-Qaeda.

Many analysts however doubt the strength of those links and say Boko Haram remains focused on a domestic agenda and is boosting its strength by exploiting religious tension within Nigeria.

The group launched an uprising in 2009 that was put down by a brutal military assault.

It fell dormant for about a year before re-emerging in 2010 and now believed to have a number of different factions, including a hardcore Islamist cell.