* Video claims recent attacks, mocks global leaders
* Boko Haram is biggest security threat in Nigeria
* Hundreds killed this year in Islamist insurgency
By Lanre Ola
MAIDUGURI, Nigeria, Sept 26 (Reuters) - A man claiming to be the leader of Nigerian Islamist sect Boko Haram in a new video said the sect took responsibility for a highway ambush that killed scores of people last week in one of its deadliest attacks to date.
Nigeria's military said last month that Boko Haram leader Abubakar Shekau may have died of gunshot wounds, weeks after a clash with soldiers, but past reports of his death have proved false and there has been no independent proof.
Although Shekau's identity could not be confirmed - he appears slightly younger and rounder faced in the latest video - it was distributed to journalists in Maiduguri through the same channels the sect always uses to diseminate statements.
"Yes ... we carried out the Benisheik attack killing more than 200 people too, we slaughtered them," says the man in the video, kneeling on a carpet scattered with rockets.
The video is the second to feature someone claiming to be Shekau since the army said he was probably dead. The man has many of Shekau's features. He has a similar beard, wears military uniform and his head is wrapped in a turban.
Nigeria's military was unavailable for comment but they denounced the previous video as the work of a Shekau impostor.
The militant group aims to revive a medieval Islamic state in Nigeria. Western governments are increasingly concerned about Islamist groups in the Sahel region, which the United States believes could become a platform for global jihadist attacks.
The sect, which is seen as the main security threat to Africa's top oil producer, killed 153 people in two roadside attacks last week near the village of Benisheik. In one, its fighters set up fake roadblocks, checking IDs, then dragging people out of their cars and killing them.
Boko Haram's insurgency against President Goodluck Jonathan's government has killed thousands since its uprising against the state in 2009. Since then, it has also forged growing links to al Qaeda's West African wing.
Statements from Boko Haram have become rare since security forces said they killed its spokesman Abu Qaqa last year. In the latest video, the man claiming to be Shekau is flanked by eight commanders who shout "Allahu Akbar" (God is greatest) and shake AK47s into the air after his pronouncements.
"I am alive by the grace of Allah. Here is Shekau. Gallantly! Dangerously!" the thickly bearded man dressed in military camouflage says in the northern Hausa language. (Additional reporting by Isaac Abrak in Kaduna; Writing by Joe Brock; Editing by Tim Cocks)