U.S. forces have launched an offensive in Iraq's al-Anbar province, and report killing dozens of insurgents. Three U.S. Marines have also been killed in the area.

U.S. military officials say the operation was launched Saturday in an area where insurgents have been hiding. "It is currently ongoing in an area north of the Euphrates River. It is a known smuggling route and sanctuary for foreign fighters, and it is aimed at eliminating the insurgents and foreign fighters from the area. It is a large offensive, mostly Marines and sailors. It is a pretty complex operation and they have been very successful so far," said multi-national forces spokesman Lieutenant Colonel Michael Caldwell in Baghdad.

A reporter for the Chicago Tribune newspaper, who is traveling with the U.S. forces, reports the operation was delayed by a counter-offensive by insurgents south of the river.

But the operations chief of the U.S. 2nd Marine Division, Colonel Bob Chase, says that counter-offensive was quickly defeated and the U.S. advance is continuing. In a telephone interview from northwestern Iraq, Colonel Chase said the operation involves well over 1,000 coalition troops, along with tanks and other armored vehicles, and both manned and unmanned aircraft. He says the operation is developing as it proceeds, based on documents the troops are finding and the interrogation of more than 20 captured insurgents.

"We have a pre-designated target set, that we have already identified, but we are finding very quickly that there are more targets available. And we are adjusting the plan on the fly as we get more viable targets and/or more lucrative targets that are being developed -- both the detainees and information that is being discovered as we attack our initial objectives."

Colonel Chase says he expects the coalition offensive in northwestern Iraq to continue for several more days, with its total duration to be determined by how many new insurgent targets it finds.