Sir Winston often visited the US, dubbing it the "great republic"

Mr Bush said threats now were different to those of Nazi Germany or the Cold War, but the need to act was the same.

"It remains the great duty of mankind to advance the cause of freedom in our time," he said.

He spoke at the launch of an exhibition on Sir Winston's life and career at the Library of Congress in Washington.

The president said Sir Winston guided his country to victory in World War II and then warned of the dangers of Communism and the Soviet empire.

'Net of doom'

"Today we are engaged in a different struggle. Instead of an armed empire, we face stateless networks. Instead of massed armies, we face deadly technologies that must be kept out of the hands of terrorists and outlaw regimes," he said.

"Yet in some ways our current struggles or challenges are similar to those Churchill knew. The outcome of the war on terror depends on our ability to see danger, and to answer it with strength and purpose.

In his determination to do the right thing and not the easy thing, I see the spirit of Churchill in Prime Minister Tony Blair US President George W Bush

"One by one, we are finding and dealing with the terrorists, drawing tight what Winston Churchill called a closing net of doom."

Mr Bush, who said he keeps a bust of Sir Winston in the Oval Office, highlighted what he called the US' "forward strategy of freedom in the Middle East", saying only a freer society there would protect Americans and others from terrorists.

"Democracies do not support terrorists or threaten the world with weapons of mass murder," he said.

The president mentioned the continuing US involvement in Afghanistan - invaded after the al-Qaeda attacks on the US - and Iraq where Saddam Hussein was overthrown amid fears that he posed a threat.

Mr Bush told an invited audience of his admiration for Britain's wartime leader of the 1940s and of his 21st Century successor.

"In his determination to do the right thing and not the easy thing, I see the spirit of Churchill in Prime Minister Tony Blair," he said.