Gunmen believed to be members of the radical Islamist group Boko Haram opened fire on a police station in Nigeria's Kano state, triggering a gun battle.

Witnesses say the gunmen arrived on motorcycles Friday evening and began shooting and police returned fire.

Police have not confirmed any injuries or deaths.

In the past week, Boko Haram has claimed responsibility for 185 deaths in Kano, Nigeria's second largest city.

The leader of the Nigeria's radical Islamic sect Boko Haram has challenged the president's vow to crush the group in an audio message posted on the Internet.

The message posted on YouTube shows a still image of Imam Abubakar Shekau flanked by Kalashnikov rifles.

In the message, he says President Goodluck Jonathan will learn he cannot defeat Boko Haram, and he warned the militant group will strike again.

President Jonathan has been under increasing pressure to bring an end to the escalating attacks, most of which have been carried out across the predominantly Muslim north. Christians dominate the south.

Mr. Jonathan has declared a state of emergency in parts of the north and this week he pressured the police chief and his deputies to retire.

Human Rights Watch this week blamed Boko Haram for more than 900 deaths since 2009.

Boko Haram has said it is fighting to implement a strict interpretation of Sharia law across Nigeria, Africa's most populous country.

The ongoing violence has raised concerns that Nigeria, which is also Africa's biggest oil producer, is sliding toward civil war.