BAGHDAD, Iraq — U.S. Marines launched a major offensive near Iraq's western border yesterday aimed at disrupting an important conduit for insurgents thought to be filtering into the country from Syria and slowing attacks before the Oct. 15 referendum on a new constitution.

About 1,000 troops stormed into the village of Sadah in armored Humvees and tanks. At least 10 people were killed by fire from helicopter gunships, said a doctor at the hospital in the nearby town of Qaim.

It is the fourth sweep this year in al-Anbar province, a wide swath of territory that is a main staging ground for attacks throughout Iraq.

Al-Anbar, with its thousands of square miles of desert, is a mostly barren tribal homeland where even Saddam Hussein failed to suppress armed movements. Insurgents shuttle between dozens of tiny villages and larger cities such as Qaim and Haditha, where Marines routinely are attacked with mortars, roadside bombs and sniper fire.

Military leaders have acknowledged that such sweeps probably won't quash the insurgency. Marines have killed hundreds of insurgents and driven them out of several towns, but as soon as U.S. forces leave, armed elements return within days.

In Washington, D.C., last week, U.S. military leaders said that reducing troop levels would push Iraqi forces toward self-reliance. And President Bush spoke yesterday of the "increasing size and capability" of Iraqi forces.

But Iraqis are not part of the new campaign, said an official with Iraq's Ministry of Interior.

"We don't have any security presence anywhere past Hit," a town about halfway between Baghdad and the site of the offensive, said the official. "Insurgents target Iraqi soldiers out there, so the operations were conducted only by American forces, without the aid of Iraqi national guards or police or any other Iraqi security forces."

The official spoke on condition of anonymity because he is prohibited from speaking to reporters.

The U.S. offensive comes two weeks before an Oct. 15 constitutional referendum that U.S. Iraqi leaders hope will stabilize the country and slow insurgent violence.

"Everybody is working to exclude Sunni Arab areas from voting in the referendum and the December elections," said Sheik Khalaf Alalian, an al-Anbar tribal leader who has negotiated for them on the constitutional drafting committee. "How can these people vote while they are scattered across the desert and their homes destroyed?"

Only 81,000 provincial residents out of more than 900,000 have registered to vote in the referendum, according to Iraqi officials.

Many Iraqis are convinced that the constitution will pass, but Sunni Arabs and some others say that it will lack enough credibility to appease violent sectarian movements.