US Secretary of Defense Donald Rumsfeld (R) and Chairman of the Joint Chiefs of Staff, Air Force General Richard Myers hold a press conference at the Pentagon,  November 8, 2004. Rumsfeld said he did not foresee large numbers of civilian deaths during the US-led assault to take the rebel-held city of Falluja. (Photo: Xinhua/Reuters)

The top US commander in Iraq on Monday predicted a "major confrontation" in the city of Fallujah as US forces launched a ground assault after a massive air strike.

"What we have generally seen is there's an outer-crust of the defense, and then our estimates tell us that they will probably fall back toward the center of the city where there will be probably a major confrontation," Gen. George Casey said in a conference call with reporters at the Pentagon.

Thousands of US Marines and Army troops as well as Iraqi security forces stormed into the toughest Sunni insurgent stronghold of Fallujah on Monday, an offensive aimed at putting anend to guerrilla control of the Sunni Muslim city.

Casey said as many 15,000 US troops and Iraqi security forces took part in the offensive.

Casey said 50 to 70 percent of the city's roughly 200,000 people had left. Defense Secretary Donald Rumsfeld said he did not expect large numbers of civilian casualties during the assault.

"The U.S. forces are disciplined," Rumsfeld told a Pentagon news briefing. "They're well-trained. They are using precision. And they have rules of engagement that are appropriate to an urbanenvironment."

But Rumsfeld declined to speculate on how many Iraqi civilians remained in the city. "There's nobody who knows how many people are in there," he said.