A man appearing to be Boko Haram leader Abubakar Shekau has dismissed Nigerian military claims of his death in a new video obtained by the AFP news agency.

"Here I am, alive. I will only die the day Allah takes my breath," Shekau said, adding that his group was "running our... Islamic caliphate" and administering strict Sharia punishments in captured towns.

The military announced last week that Shekau was dead and that a man who had been posing as the group's leader in the earlier videos had been killed after fighting with troops in the far northeast.

But security analysts and the United States questioned the credibility of the military's claim.

Meanwhile, the Reuters news agency stated that it had seen screen grabs of the video and that the man looked different from before, with a wider nose, less defined bridge and a rounder face compared to Shekau.

Previous claims

The new 36-minute video shows Shekau, in combat fatigues and black rubber boots firing an anti-aircraft gun into the air.

Standing in a pick-up truck in front of three camouflaged vans and flanked by four heavily armed, masked fighters, he then speaks for 16 minutes in Arabic and the Hausa language widely spoken in northern Nigeria.

There was no indication of where or when the video was shot.

There have been two previous claims by Nigeria's security forces that Shekau had been killed or "may be dead" but Boko Haram has later issued denials in video messages.

Boko Haram has shown images of extreme violence before but the latest video, according to the AFP, shows at length graphic scenes of an amputation and a stoning to death as well as a beheading.

It also purports to show the wreckage of a Nigerian Air Force jet that went missing in the northeast on September 11.

Boko Haram said its fighters shot it down but the military has denied the claim.