U.S. President George W Bush said Thursday that al-Qaida was bent on building a "totalitarian empire" grounded in radical Islam, and put its leader Osama bin Laden on a par with Adolf Hitler, Pol Pot and Joseph Stalin.

In a speech on terrorism, Bush said it was a "dangerous illusion" that the United States would be better off pulling troops out of a conflict which has cost nearly 2,000 U.S. lives and hammered his personal opinion poll ratings.

Bush compared terrorist leaders to ideological "fanatics" Soviet leader Joseph Stalin, Nazi tyrant Adolf Hitler, and Cambodia's Khmer Rouge kingpin Pol Pot.

"Evil men obsessed with ambition and unburdened by conscience must be taken very seriously and we must stop them before their crimes multiply," Bush said.

"We have seen this kind of shameless cruelty before, in heartless zealotry that led to the gulags and the Cultural Revolution and the Killing Fields."

Speaking in Washington's Ronald Reagan Building, dedicated to the man many Americans believe was instrumental in winning the Cold War, Bush made several comparisons between al-Qaida and communism.

"Like the ideology of communism, our new enemy pursues totalitarian aims. Its leaders pretend to be an aggrieved party, representing the powerless against imperial enemies.

"In truth, they have endless ambitions of imperial domination, and they wish to make everyone powerless except themselves."