A group of Uzbeks in northern Afghanistan, claiming to be from the Islamic Movement of Uzbekistan (IMU), says it is pledging allegiance to the Islamic State extremist group.

A person calling himself Sadulla Urgenji said the IMU no longer views Taliban leader Mullah Omar as leader since he has not been seen for some 13 years and, "according to Shari'a," can no longer be leader.

Urgenji said his group was recognizing the authority of the Abu Bakr al-Baghdadi, leader of the Islamic State group.

The group also posted a video online that showed members beheading an Afghan soldier.

Urgenji said Afghan forces had captured several female IMU members in Faryab Province recently and in retaliation the group took 30 Afghan soldiers captive.

He said Afghan authorities ignored an offer to exchange the soldiers for the IMU women supporters so the group decapitated one of the soldiers and promised to execute more unless their demand was met.

Afghan authorities have not commented on the video.