Israeli troops seeking to quell protests over Israel's West Bank separation barrier on Friday sprayed a foul-smelling substance code-named "skunk" and sent in undercover troops posing as Palestinians.

The use of undercover troops is rare in such circumstances, and the military said it was just the second time security forces had used "skunk," which it said was a non-lethal substance meant to reduce injury to protesters.

Confrontations over the barrier have become increasingly violent, with two Palestinian demonstrators killed in recent months and an American supporter seriously injured.

Video footage of one incident at the West Bank village of Naalin showed three masked undercover agents surrounding a shirtless Palestinian demonstrator, throwing him to the ground and then calling for backup by uniformed soldiers.

Several rocks hurled by protesters struck the ground near the troops and one of them pulled a pistol and fired in the air as Palestinians fled the scene. Two Palestinians were arrested, the military said.

It had no comment on the use of undercover forces. At least half a dozen plainclothes agents were at the scene.

In nearby Bilin, troops sprayed a putrid mist that stuck to protesters' clothes. The military said the substance was non-toxic and not life-threatening and that its use was ordered by the top commander in the area to reduce injury to protesters. It said the mist was used once before, nearly a year ago.

Palestinians have been staging weekly demonstrations at both sites to protest the barrier's route, which crosses through the villages and cuts farmers off from hundreds of acres of agricultural fields.

Israel says the barrier _ a mix of towering concrete walls and electronic fences _ is needed to stop Palestinian militants from crossing into Israel to carry out attacks. Palestinians call it a land grab because parts of it jut deep into the West Bank, cutting them off from territory they claim for a future state.

Israel began building the nearly 500-mile (800-kilometer) barrier in 2002 and it is about two-thirds complete.

Israel has classified the protest areas as closed military zones and troops have clashed frequently and increasingly violently with protesters, some of whom hurl rocks at the soldiers.

In May, Israel's Justice Ministry opened a criminal investigation into the firing of tear gas shells at the demonstrations. Tear gas shells struck one of the two Palestinian demonstrators killed recently and injured the American.

Deputy state prosecutor Yehoshua Lamberger said at the time that it appeared troops made "illegal use" of the weapons during the protests, firing the tear gas shells directly at the protesters. The shells are designed to be non-lethal when used properly, he said.

The statement also said four other cases that occurred before the first demonstrator was killed in April are now being investigated, and criminal inquiries have been opened against police officers involved in those cases.

A series of Israeli Supreme Court rulings has forced the government to reroute the barrier closer to the cease-fire line that separates the West Bank from Israel.