By Ardo Abdullahi

BAUCHI, Nigeria (Reuters) - The purported leader of Nigerian Islamist militant group Boko Haram Abubakar Shekau appeared in a video posted on social media on Sunday in which he rejected statements by the country's military that he had been seriously wounded.

In recent years, the Nigerian military has said it has killed or critically wounded Shekau on multiple occasions, often swiftly followed by video denials by someone who says he is Shekau. Last month Nigeria's air force said it had killed senior Boko Haram members and that Shekau had been wounded.

While the ensuing videos all show someone sporting Shekau's distinctive beard, the grainy quality of the footage means it is not always possible to confirm if the person is the same as in the previous videos.

"You broadcast the news and published it in your media outlets that you injured me and killed me and here I am," said a man purporting to be Shekau in a video addressed to "tyrants of Nigeria in particular and the west of Africa in general".

"I will not get killed until my time comes," he added in the 40-minute video posted on YouTube and delivered in Arabic and Hausa, which is spoken widely in northern Nigeria.

A statement issued by army spokesman Sani Usman said the footage showed that the man purporting to be Shekau was "unstable" and came as "another sign that the end is near for him".

"Boko Haram terrorism as it was known, is gone for good. We are just counting down to the day when all the few remnants will be totally wiped out or brought to justice," he said.

The statement did not explicitly say whether the army considered the man in the video to be Shekau.

Last month's announcement by the air force came days after Islamic State, to whom Boko Haram pledged allegiance last year, announced the appointment of a new leader of the West African group in an apparent rejection of Shekau.

That appointment was later dismissed in a 10-minute audio clip on social media by a man purporting to be Shekau, exposing divisions within the jihadist group that has plagued Nigeria and neighbors Chad, Niger and Cameroon.

Boko Haram has killed about 15,000 people and displaced more than 2 million in a seven-year insurgency aimed at creating a state adhering to strict Islamic laws.

It controlled a swathe of land in northeast Nigeria around the size of Belgium at the end of 2014 but was pushed out by Nigerian troops, aided by soldiers from neighboring countries, early last year.

In a sign the group remains capable of inflicting damage, Usman said four Nigerian soldiers were killed and 16 others were injured on Sunday when they were ambushed by Boko Haram fighters during a patrol in the northeastern state of Borno. He said three militants were killed.

It comes hours after two security sources told Reuters in Chad that suspected militants killed four Chadian troops and wounded six overnight in an attack near the town of Kaiga.

They added that seven of the Islamist militants had been killed in return fire.

(Additional reporting by Muath Freij in Jordan and Madjiasra Nako in N'Djamena and Felix Onuah in Abuja; Writing by Alexis Akwagyiram and Tim Cocks; Editing by David Goodman and Sandra Maler)