"I cannot imagine that it would stop without being completed," Defence Secretary Donald Rumsfeld said at a Pentagon news conference Monday.
Arguing that it was impossible to have a free, democratic nation where terrorist safe havens exist, he said: "It will take time to return Fallujah to the control of the interim Iraqi government, but it is obvious that terrorist safe havens cannot be allowed to exist in Iraq."
Iraqi Prime Minister Iyad Allawi had tried to find a peaceful solution to the situation in Fallujah, but when his overtures failed he decided to use force, Rumsfeld said.
"Iraqi security personnel and coalition force are working together to oust the insurgency in Fallujah. Success in Fallujah will deal a blow to the terrorists in the country and should move Iraq away from a future of violence to one of freedom and opportunity for the Iraqi people,"he said.
There are more than five Iraqi military brigades working with US forces, and they are being supported by British forces. A decision was made to go into the city jointly and the forces involved will also finish together, he said.
The secretary said the military assault on Iraqi city would be carried through to completion unlike the US Marine operation in April that was aborted after several days.
Asked about the risks to the civilian population in Fallujah, Rumsfeld said he did not expect large numbers to be killed as US military forces are skilled in urban combat and are using precise and carefully targeted munitions.
Asked if success in Fallujah would put an end to what Defence Department officials call the "dead enders" in Iraq, Joint Chiefs of Staff Richard Myers, who was also present at the conference, said: "time will tell and this will not be the last use of force in Iraq to rid the country of foreign fighters who do not want Iraq to be successful."
Myers said destroying the major safe haven of former regime elements and foreign fighters, including Abu Mussib al-Zarqawi and his followers will be of great significance.
Even if some of them slip away and fight from other locations it will affect their ability to function, he said.