"Allegations such as this, regardless of how they are borne out by the facts, can have an effect on the ability of U.S. forces to continue to operate," said Ham, a deputy operations director for the Joint Staff and a former commander of U.S. forces in northern Iraq.

"We do rely very heavily and more importantly, the Iraqi security forces rely heavily on the support from the Iraqi people," Ham said. "And anything that tends to diminish that, obviously, is not helpful to what we're trying to do."

The toll of Iraqi civilians climbed on Wednesday when two women, including one being taken to a maternity hospital, died when coalition troops shot at a car that failed to stop at an observation post in a city north of Baghdad. The U.S. military said the vehicle entered a clearly marked prohibited area but failed to obey repeated warnings.

The president was asked about the Haditha allegations during a photo opportunity with the president of Rwanda, Paul Kagame.

Bush said he had discussed Haditha with Gen. Peter Pace, chairman of the Joint Chiefs of Staff. "He's a proud Marine. And nobody is more concerned about these allegations than the Marine Corps. The Marine Corps is full of honorable people who understand the rules of war."

"If in fact these allegations are true," Bush said, "the Marine Corps will work hard to make sure that that culture that proud culture will be reinforced. And that those who violated the law, if they did, will be punished."

Until now the most infamous violation of military law in Iraq was the Abu Ghraib prisoner abuse involving Army soldiers, which came to light in April 2004. Bush said last week he considered Abu Ghraib to be the most costly U.S. mistake of the war.