Gunmen opened fire on a police station Friday and two officers were feared dead in the Nigerian city of Kano, where attacks claimed by Islamist group Boko Haram left 185 dead last week.

The latest attack occurred just before 7:00 pm, the start of a nighttime curfew in effect since last week's attacks, residents said.

"Gunmen came on motorcycles and one all-terrain van and opened fire on the police station," a resident said. "It was quite brief, lasting not more than five minutes ... Two policemen lay in pools of blood."

The resident said the officers appeared to have been killed, but their condition could not be immediately confirmed. Another resident reported gunshots but could not say whether any officers had been hit.

Both claimed they heard the gunmen shouting "Allahu Akbar."

The attack happened in the Mandawari neighbourhood not far from the palace of the emir of Kano, the most important traditional leader in the city.

Boko Haram was blamed for coordinated bombings and shootings in Kano, Nigeria's second-largest city, on January 20 that killed at least 185 people, mainly targeting police stations.

Another police station was attacked on Tuesday night, with authorities reporting three people wounded. On Thursday, a German engineer was kidnapped on the outskirts of Kano.

A blast also hit a bus station in Kano on Thursday, but no one was reported killed.

The purported head of Boko Haram said in an Internet message that he ordered the gun and bomb attacks that rocked Kano last week, the deadliest assault attributed to the shadowy group.

"We were responsible," said Abubakar Muhammad Shekau in the audio message posted on YouTube. "I ordered it and I will give that order again and again. God gave us victory."

The authenticity of the Hausa language message could not be independently verified but the photo appeared to match others said to be of the Boko Haram leader.

The violence sparked fear in the city, the economic heart of Nigeria's mainly Muslim north.

"Honestly I went to the mosque in fear," said Isa Bello, 58, after leaving Friday prayers. "We ardently hope for unity and understanding between Muslims and Christians in Nigeria."

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

The group was also blamed for coordinated attacks on Christmas Day, the mostly deadly at a Catholic Church near the capital Abuja where at least 44 people were killed, but its victims also include scores of Muslims.

Nigeria's Vice President Namadi Sambo on Friday denied that religious tensions were fuelling the Boko Haram menace in the country whose population is roughly divided between a predominantly Muslim north and a mainly Christian south.

"It is very clear that there is no religious problem, religious fighting in northern Nigeria," Sambo told journalists after a meeting with governors from 19 northern Nigerian states.

Elizabeth Donnelly, an Africa analyst at the London-based think-tank Chatham House, said such tensions do exist and that Boko Haram may be using them to boost their strength.

"If they thought it was to their advantage, it would be quite easy for Boko Haram to exploit existing tensions," she told AFP.

There has also been intense speculation about Boko Haram's links to foreign Islamist groups, specifically Al-Qaeda's north African affiliate, known as Al-Qaeda in the Islamic Maghreb (AQIM).

A Western diplomat downplayed the strength of those ties.

"I think there's evidence of contact (with foreign groups), but in terms of operationally linking up with AQIM or extremist groups elsewhere, we don't see Boko Haram as an Al-Qaeda franchise," said the diplomat who requested anonymity.

Shekau was seen as Boko Haram's second-in-command at the time of a 2009 uprising put down by a brutal military assault, after which the group went dormant for about a year before re-emerging in 2010.

In his latest Internet post, he denied targeting civilians in last week's attacks, blaming civilian deaths on the police.