Washington, Sep 12 (DPA) US President George W. Bush called for unity in confronting worldwide terrorism on the fifth anniversary of the Sep 11, 2001, attacks and reminded Americans of the challenges that remain.

Tapping into the emotional displays during a series of ceremonies to mark five years since Al Qaeda hijackers slammed planes into the World Trade Center and Pentagon, Bush cast the war on terrorism as an ideological struggle that will define the 21st century.

"If we do not defeat these enemies now, we will leave our children to face a Middle East overrun by terrorist states and radical dictators armed with nuclear weapons," he said in a primetime televised address.

"We are in a war that will set the course for this new century - and determine the destiny of millions across the world."

In the days leading up to Monday's commemorations, US television stations recounted the events that shook the world, re-broadcasting footage of the towers collapsing and the Pentagon being struck by an airliner.

Nearly 3,000 people died that day.

Bush faces persistently low approval ratings, and polls show the US public's confidence sagging in the president to conduct the war on terrorism and the conflict in Iraq as his Republican party heads into crucial mid-term elections Nov 7.

Bush's speech was aimed at rallying the public behind his policies, which evolved out of the worst attack on US soil since World War II.

"Our nation has endured trials - and we face a difficult road ahead," he said. "Winning this war will require the determined efforts of a unified country."

Bush defended his decision to invade Iraq, saying decades of promoting stability in the Middle East had failed to rid the world of violence, and that it was now necessary to spread freedom and bring democratic change to the region.

The war in Iraq has continued to dog Bush as the US death toll there approaches 3,000, amid a bloody insurgency and concerns of possible full-blown civil war. Polls show most Americans no longer support the war in Iraq.

"Whatever mistakes have been made in Iraq, the worst mistake would be to think that if we pulled out, the terrorists would leave us alone. They will not leave us alone. They will follow us," he said from the Oval Office.

While warning that the US remains vulnerable to Al Qaeda attacks, Bush touted his administration's record of preventing terrorism at home during the last five years.

"Five years after 9/11, our enemies have not succeeded in launching another attack on our soil," he said. "But they have not been idle."

Bush emphasised that terrorists seek to repress the entire region.

"The war against this enemy is more than a military conflict," he said. "It is the decisive ideological struggle of the 21st century and the calling of our generation."