The United States urged Sri Lanka on Saturday to investigate the recent killing by soldiers of unarmed protesters who were demanding clean drinking water.

Three youths were shot dead and at least 33 people wounded on Aug. 1, when the military fired at demonstrators in Weliweriya, a village some 30 km (20 miles) north of Colombo, who were protesting against factory waste contaminating ground water.

President Mahinda Rajapaksa's government has ordered the army to conduct a prompt investigation into the incident, during which, the protesters say, the military attacked them as they took refuge in a church.

"We are particularly concerned by reports that protesters seeking refuge within a Catholic church were attacked there. There is never any excuse for violence, particularly in a house of worship," U.S. State Department spokeswoman Jen Psaki said in a statement.

"We call for a thorough and transparent inquiry into all aspects of the Weliweriya violence, for those conclusions to be made public, and for there to be a credible mechanism to prosecute any wrongdoing."

The government has said the soldiers reacted in self-defence against protesters hurling rocks and petrol bombs, violence it said had been incited by "external forces".

"It's undisputed that security forces killed three protesters, but the Sri Lankan government's knee-jerk reaction is to deny possible wrongdoing," said Meenakshi Ganguly, South Asia director at New York-based advocacy group Human Rights Watch.

Army Commander Daya Rathnayake has appointed a five-member committee to look into the incident, but Amnesty International said the military should not be allowed to investigate itself.

President Rajapaksa is already under pressure from Western countries and the United Nations over alleged human right abuses during the last phase of a civil war that ended in May 2009 with the military annihilating separatist Tamil Tiger rebels.

Rajapaksa last month ordered an inquiry into mass disappearances during the war, ahead of U.N. High Commissioner for Human Rights Navi Pillay's visit later this month and a Commonwealth Heads of Government Meeting in November.