Bush promised that if an ongoing U.S. military investigation finds that laws were broken, the Marines involved will be punished.
This is Bush's first public comment on allegations that Marines killed about two dozen Iraqis in the western city of Al-Hadithah last November.
U.S. officials on May 30 said they will make public the results of investigations into the 24 deaths.
Iraqi Prime Minister Nuri Kamil al-Maliki said his government would also investigate the allegations, and added that there is a limit to what he called "acceptable" excuses for civilian casualties.
The problems for the U.S. military were compounded today when a joint U.S.-Iraqi security body said U.S. forces recently shot and killed two women by mistake as the two were driving to a maternity hospital.
The security body said coalition U.S. forces fired at the two women in Samarra, north of Baghdad, when they drove into a prohibited area and failed to stop despite repeated warnings.