By Chukwuemeka Madu

KANO, Nigeria (Reuters) - Nigerian troops have killed Islamist sect Boko Haram spokesman Abu Qaqa, long the shadowy group's public messenger in its campaign to impose sharia law on Nigeria, in a gun battle in the northern city of Kano, a senior security source said on Monday.

The Nigerian security source, who spoke on condition of anonymity, said the man killed had been identified as Abu Qaqa, whose name frequently appears in pronouncements on behalf of the sect from its northeastern base.

He said that another top Boko Haram figure may also have been killed and that at least one fighter was arrested.

A spokesman for the military and police joint task force in Kano, Lieutenant Ikedichi Iweha, said checks were being made to determine the identities of the three men, two of whom he said had been captured alive.

"I know one person was killed in a gun battle while two others were captured alive, but as for their identities, for now we don't know," Iweha said.

Qaqa has been reported killed or captured before. Nigerian authorities in March claimed to have arrested him, only to have him appear publicly to deny it. The man they caught turned out to be another of the sect's members.

The name is probably a pseudonym, complicating efforts to verify his identity, Nigerian security sources have said.

"You might say the man killed is Qaqa, then next moment you hear another Qaqa on air. So we are being careful," said Iweha.

The Islamists, who are fighting to impose sharia or Islamic law across parts of Nigeria, a country of 160 million people with a roughly fifty-fifty mix of Muslims and Christians, have killed hundreds since they launched an uprising in 2009.

A military crackdown on the group in the north appears to have weakened their capabilities, and they have not managed a deadly large-scale strike for several months, but the group remains active across the north.

Boko Haram's fighters usually target security forces, government offices and churches. They started attacking mobile phone installations about a week ago, saying phone companies were helping authorities to track them.

On Sunday, suspected Islamists on motorcycles shot dead an off-duty military officer, his wife and their disabled child, police said.

If the militant killed is confirmed as Abu Qaqa, a deputy commander, it is not clear what impact this would have on the shadowy command structure of the sect, which the United States added to its list of what it terms foreign terrorist organizations in June.

A man calling himself Abu Qaqa often appears in local media or speaks to journalists by telephone to claim responsibility for gun or bomb attacks for the group, justify its actions or deny things attributed to them, or to threaten the media.

For a long time Abu Qaqa was the closest thing the sect had to a public figure, before its purported leader Abubakar Shekau began posting videos of himself on YouTube in January.

(Reporting by Chukwuemeka Madu; additional reporting by Camillus Eboh and Joe Brock; writing by Tim Cocks; Editing by Jason Neely and Roger Atwood)