JERUSALEM - Israel's prime minister Monday rejected U.S. calls for a settlement freeze as unreasonable, moving closer to a collision with the Obama administration, while mobs of Jewish settlers attacked Palestinian labourers and burned West Bank fields.

Six Palestinians were injured in the stone-throwing attacks, meant to protest the removal of several tiny settler squatter camps by the government of Benjamin Netanyahu.

Monday's events highlighted Netanyahu's increasingly difficult balancing act. The hardline leader is trying to keep his pro-settler ruling coalition together by rejecting President Barack Obama's call for a halt to all settlement activity, at the risk of hurting Israel's all-important relationship with the United States.

In an apparent gesture to Obama, Netanyahu has begun dismantling small settler outposts built without formal government authorization. But even that limited step risks triggering settler violence against Palestinians and further international criticism of Israel.

Settlers have vowed to respond with attacks on Palestinians and their property to any attempt to remove even the tiniest enclave -- a tactic known as "price tag."

"We will do everything we can to oppose this," said Yehuda Shimon, a resident of the Havat Gilad outpost in the northern West Bank.

In Jerusalem, Netanyahu briefed the Israeli parliament's Foreign Affairs and Defence Committee about his recent meeting with Obama at the White House. The American president and his secretary of state, Hillary Clinton, have demanded that Israel halt all settlement construction, including expansion to accommodate what Israel calls "natural growth" of settler communities.

Netanyahu said Israel cannot "freeze life" in settlements, according to a participant who spoke on condition of anonymity because the meeting was closed. Netanyahu was quoted as saying that "there are reasonable requests and unreasonable requests."

Monday's settler violence started near the radical settlement of Yizhar, police spokesman Micky Rosenfeld said. About 100 settlers blocked a road to protest Israel's recent removal of a handful of tiny, uninhabited outposts. Six settlers were later arrested.

Before dawn, near the Kedumim settlement, stone-throwing settlers ambushed a minivan carrying Palestinian labourers to Israel, the workers said. Six of the 15 Palestinians on board were hurt, including Yahye Sadah, 44, who was hit in the head and said he got six stitches.

Police said settlers threw rocks and burned tires in the area. The attackers fled and no arrests were made, they said.

A few hours later, settlers torched a wooded hilltop near Nablus and set trees and Palestinian agricultural land on fire near the village of Hawara, residents said. Romel Sweiti, a Hawara resident, said about 50 teenage settler girls gathered on a main road and blocked traffic as Israeli paramilitary police stood in the background.

Toward nightfall, about 20 young Jewish extremists briefly blocked the highway into Jerusalem, burning tires and a garbage bin, causing a huge rush hour traffic jam. Police dragged the struggling youths away, arresting four.

Nearly 300,000 Israelis live in the settlements among 2.4 million Palestinians in the West Bank. Another 180,000 live in Jewish neighbourhoods of east Jerusalem. The Palestinians claim both areas -- captured by Israel in the 1967 Middle East War -- as parts of a future independent state.

In recent years, settlers have set up dozens of squatter camps, or so-called outposts, that lack formal government approval, but often received funding and support from government agencies. Israel has failed to keep a promise to the U.S., first made in 2003, to dismantle about two dozen outposts.

The U.S. considers the settlements an obstacle to peace, but traditionally has done little on the issue, a policy that appears to be changing under Obama.

Netanyahu has dispatched his defence minister, Ehud Barak, to Washington this week in hopes of winning approval to allow at least limited construction to continue in the settlements, apparently in exchange for removing outposts. But the Obama administration has so far signalled it is not willing to budge.

In another possible diplomatic entanglement, United Nations investigators Monday began looking into possible war crimes during Israel's three-week offensive against Gaza's Hamas rulers, even though they failed to secure a promise of co-operation from Israel.

Israeli officials have insisted the investigation, led by veteran war crimes prosecutor Richard Goldstone, would not be objective, citing alleged anti-Israel bias by the UN agency sponsoring it.

Goldstone, who is Jewish and has close ties to Israel, has said he wants to investigate both Israel and Hamas. He said Monday, after arriving in Gaza City with a 15-member team, he would deliver his report by August.