Iraqis marched through the Sunni stronghold of Fallujah, where U.S. Marines spent another day battling anti-American insurgents in a major operation to hunt down those responsible for the grisly deaths of four American civilians last week.

Reports from the city say a coalition helicopter fired on a mosque filled with worshippers gathered for afternoon prayer, causing a number of casualties. But a statement from U.S. military headquarters in Baghdad denied reports of civilian casualties. U.S. General Mark Kimmitt told CNN there were a number of what he called enemy casualties after two precision-guided bombs damaged an outer wall of the mosque after Iraqi insurgents began firing from the compound and inciting violence.

U.S.-led coalition forces are now involved in the most widespread fighting in Iraq since the fall of Saddam Hussein a year ago. Troops battled Shiite militias in half a dozen Iraqi towns and cities from near Kirkuk in the north to Basra in the south.

While saying U.S. troops still control the country, Defense Secretary Donald Rumsfeld told reporters at the Pentagon the future of the Iraqi people is at stake. "We will take robust military action as necessary to deal with the challenges to Iraq's transition to sovereignty," he said. "There are those who don't want Iraq to be free, they're trying to stop progress toward freedom and self-government. We will not allow them to succeed."

But Mr. Rumsfeld says at least one town, Najaf, is no longer under the control of coalition forces. Troops pulled out ahead of an upcoming Islamic holiday in order to reduce the possibility of clashes with Shiite pilgrims who are expected to flock there. "We caution all pilgrims that the holy cities are potentially dangerous places during this period," he added.

The Shiite unrest gathered force after the U.S. military closed a Shiite newspaper last month and after influential Shiite cleric, Moktada al-Sadr, urged his followers to rise up against the U.S.-led occupation.

It's unclear how much support exists for the Shiite rebellion among the Iraqi population, but it amounts to the most widespread unrest since the end of major combat operations nearly a year ago. Also unclear is the extent of coalition casualties in the recent fighting. In one attack alone Tuesday night, at least 12 American Marines were killed when their base in the Sunni stronghold of Ramadi came under attack.

From his ranch in Texas, President Bush was briefed on the fighting through a special video hook up with the top American commander in the region. This, amid growing concern about the war in Washington reflected in these comments by Senator Joe Biden, another Democrat who is now comparing Iraq to Vietnam. "The marching that is taking place, the uprising that is occurring in the (Sunni) triangle as well as that portion of the Shiite community is communicating the similar fear to the American people: A. We're alone, we're the only ones in on the deal. B. We don't have a plan," he said.