A UN expert urged Mexico on Monday to conduct an independent investigation into the killing of 22 drug gang suspects by soldiers, saying the deaths may have been summary executions.

Christof Heyns, the United Nations special rapporteur on extrajudicial executions, welcomed the military's arrest last week of seven soldiers and one officer "in what could be summary executions," said a statement by the UN's human rights office.

"The government of Mexico has the duty to fully investigate, prosecute, and punish all suspected cases of extra-legal, arbitrary and summary executions," Heyns said in the statement.

Heyns called on the government to protect a witness whose testimony contradicted official accounts that the suspects had died in a gunfight with soldiers on June 30 in Tlatlaya, 240 kilometers (150 miles) southwest of Mexico City.

The witness told Esquire magazine that 21 of the suspects, including her 15-year-old daughter, were executed after surrendering, while only one person died during a shootout.

Heyns said the authorities should also protect two other women who survived the shooting and were detained on charges of firearms possession and organized crime.

The federal attorney general's office is conducting its own investigation into the case, which has put a spotlight on Mexico's controversial use of the military in the drug war.