KABUL- At least five Afghan civilians were wounded when a combined force of Afghan troops and U.S. Marines opened fire on a crowd at the gate to a military base in Helmand, Afghanistan's most volatile province, NATO said on Friday.

The incident, which took place on Wednesday but was not reported until Friday, was the second demonstration to turn violent in two days in Helmand's Garmsir district, suggesting mounting civil unrest in a part of the country where U.S. Marines under NATO command made major advances last year.

ANA and ISAF forces warned a crowd of between 200 and 400 assembled civilians to keep its distance from the outpost, a NATO statement said, referring to the Afghan National Army and NATO-led International Security Assistance Force.

ISAF is manned in the area by U.S. Marines.

A number of civilians in the crowd disregarded instructions, resulting in forces firing warning shots. Deliberative escalation of force procedures were followed, but one individual continued to ignore instructions, striking members of the combined force with a stick, the statement said.

Lieutenant-Colonel Todd Breasseale said both Afghan troops and the U.S. Marines subsequently fired at the crowd. An investigation was under way to determine which force's bullets had struck each the five people who were wounded.

Civilian casualties caused by NATO troops are one of the most emotive issues in Afghanistan's eight-year-old conflict.

The incident came a day after another violent demonstration in Garmsir. During that earlier demonstration, U.S. Marines say they fired only at a sniper, who had shot into their base. Afghan officials say Afghan troops killed eight protesters and wounded 13 who were trying to storm a government building.

Afghan and U.S. officials say the initial unrest was prompted by rumors that U.S. troops had defaced a holy book during a raid. U.S. and Afghan officials met with locals in the area to restore calm and deny the rumors in strong terms.

A lot of this came from a massive Taliban-initiated hoax, Breasseale said. People started behaving dangerously and unfortunately things like this happen.

Dawood Ahmadi, spokesman for Helmand governor Gulab Mangal, said Wednesday's demonstration had taken place outside a base where U.S. and Afghan officials were discussing the unrest from the day before.

He said Taliban infiltrators in Wednesday's crowd fired at the U.S. and Afghan troops, prompting the Afghans to return fire. The NATO statement made no mention of shots fired from the crowd.

Garmsir is located in the lower Helmand River Valley, most of which was seized by U.S. Marines in July in the biggest offensive of the eight-year-old war.

The Garmsir district center is one of the calmer parts of the valley, but Taliban influence remains strong in outlying villages along the river, part of Afghanistan's main opium-growing region.

(Additional reporting by Hamid Shalizi; Editing by Sanjeev Miglani)

Afghanistan