Islamic State have purportedly executed British aid worker Alan Henning, releasing a video on Friday showing him kneeling beside a militant in a desert setting.

According to the SITE Intelligence Group, which monitors jihadist groups, the one minute, 11-second long video titled "Another Message to America and its Allies" showed Mr Henning introducing himself and saying "because of our parliament's decision to attack the Islamic state, I, as a member of the British public, will now pay the price for that decision."

The United Kingdom commenced air strikes against IS in Iraq earlier this week.

The UK Foreign Office has said that it is working urgently to verify the contents of the video.

Kenyan President Uhuru Kenyatta will have to appear at a hearing next week, after the International Criminal Court rejected his request to postpone it or appear via video link.

"The chamber, by majority, finds that the requirements of justice in this case necessitate the physical presence of the accused in court," the ICC said in a statement.

Judges at the court added that the matters to be discussed on October 8 were at a critical stage and directly involve the interest of the accused and the victims.

Mexico has charged three of its troops with murder, after a shootout in June, which left 22 suspected gang members dead.

Attorney General Jesus Murillo said the soldiers shot the men "without any justification whatsoever".

The victims are alleged to be members of a notorious criminal organisation called La Familia Michoacana and the army at the time said the men died in a fire fight.

However, some the victims appeared to have bullet wounds in the chest, inconsistent with a shootout.