Mexican authorities have arrested an army officer and seven soldiers over the killing of more than 20 suspect gang members amid claims they were summarily executed.

The Mexican Defence Department said the servicemen are being held at a military prison in Mexico City over a June incident in the rural town of Tlatlaya, south west of the capital.

They are facing charges of crimes against military discipline, disobedience and dereliction of duty over the June shooting.

Initially the army claimed that 22 gang members - all teenagers or in their early 20s, were shot dead during a fierce gun battle with security forces.

The official account was disputed after it emerged that only one soldier was wounded and AP reporters visited the site, unveiling some inconsistencies.

The journalists said there were no signs of a prolonged battle, while blood and bullet marks inside a warehouse where the bloodshed occurred suggested at least five people had been shot in the chest from a close range while standing against a wall.

Later a woman said she saw the soldiers shooting her 15-year-old daughter more than half a dozen times as she lay on the ground injured.

"A soldier stood the kid up and killed him," the mother who spoke on condition of anonymity said.

She said that only one gang member was killed and several wounded during the initial shootout. The remaining 21 people were shot dead after surrendering, she said.

Human Right Watch called for a throughout and independent investigation to be carried out saying that the incident could prove to be one of the "most serious massacres in Mexico".

In July, the state of Mexico prosecutors' said there was "no evidence at all of possible executions," and last week the federal Attorney General's Office added that so far no evidence corroborating the witness' account was found.

The incident is now being investigated both by the military and by Mexico's National Human Rights Commission, so that the arrested soldiers might face an additional civil trial.