Pakistan authorities are investigating a video which appears to show the shooting of an unarmed man by five members of the security forces in Karachi.

Major General Aijaz Chaudhry, the chief ranger for Sindh province, said the inquiry was set up after footage of the alleged killing on Thursday of Afsa Shah, 18, appeared on local television stations and the internet website, YouTube.

Shah is alleged to have attempted to steal from a policeman's family in Clifton, Karachi's most exclusive neighbourhood, on Wednesday.

Shah's brother, Salik, a local TV crime reporter, denied Afsar was a robber and accused the the security forces, also known as rangers, of shooting an innocent person.

"My brother has been killed extra judicially. My innocent brother has been killed brutally by the rangers," said Salik. "What harm has he inflicted on anybody?"

The footage of the alleged brutal killing of the man has outraged Pakistanis.

Many people in Pakistan are already losing confidence in the national military, following the US killing of Osama bin Laden in Pakistan and recurring fatal US drone attacks within Pakistan's borders.

The graphic video appears to depicting the young man pleading for his life before he is shot in the hand and thigh by a soldier and then left to die.

The footage sparked anger from the hundreds of mourners that attended Shah's funeral.

Many chanted "the rangers are killers" and demonstrators in the streets of Karachi demanded "hang the killers".

Rehman Malik, the Pakistan interior minister, said: "This act is extremely unlawful, even if the youth was a robber it did not merit to kill him like this."

Police official Tariq Dharejo told the AFP news agency that the security forces were in rangers' custody but would be handed over to police and that legal proceedings would begin after an internal inquiry.

Last month, security forces shot dead five unarmed Chechens at a checkpoint near the southwestern city of Quetta. One of them was a pregnant woman.

Officials initially claimed the five were suicide bombers, but footage showed them to be unarmed and dispelled the government's claim.

Chairman of the Human Rights Commission of Pakistan, Zohra Yusuf, said the country had descended into a "trigger-happy society where shoot-to-kill has become routine practice for the law enforcement agencies".