The number of U.S. troops in Iraq should not be so big that they appear to be an occupying force, despite recent spikes in sectarian violence, Defense Secretary Donald Rumsfeld said Thursday.

Speaking at the Harry S. Truman Presidential Museum and Library, Rumsfeld said the United States has to "avoid filling every vacuum" in Iraq. He said America needs to have enough military forces there to support the fledgling government and the development of a new political system, but not so many that it will feed the insurgency and make Iraqis think the U.S. is there only for oil.

There are 133,000 U.S. troops in Iraq, a figure officials have said they would like to pare below 100,000 by year's end. Rumsfeld used no numbers in his remarks.

Rumsfeld also said that America's impatience and political division over the war on terror mirrors disagreements during the Cold War.

He said that during the Cold War the nation was tired after World War II and not in the mood "for more global involvement."

During the Cold War, he said, "the future then too was unclear, the tasks often seemed insurmountable, and it was difficult to view things with the perspective that only history can offer."

He said President Truman and other American leaders pressed ahead in the confrontation against communism, bolstering democracies in Western Europe.

While today's enemy is a "shadowy movement of terrorist cells," not an empire, Rumsfeld said it is an equally long war that will require Americans to gird for a sustained struggle.

The surge in violence in Iraq was triggered by the Feb. 22 bombing of a sacred Shiite shrine in Samarra. U.S. officials have been struggling to hold together negotiations between Sunni, Shiite and Kurdish leaders aimed at forming an inclusive government in the war torn country.