NEW YORK - United States President George W Bush grimly vowed to avenge September 11's victims as he today honoured the dead at the three sites where hijacked aircraft crashed one year ago, killing more than 3000 people.

Bush arrived on Wednesday afternoon (local time) in New York, the last stop of his solemn anniversary pilgrimage, to visit the Ground Zero site of the collapsed World Trade Centre. The centre's twin towers fell after being hit by two of the four airliners hijacked by young Arab men in a carefully co-ordinated attack.

Accompanied by his wife, Laura Bush, he laid a wreath in the empty space that once contained the twin towers, and hugged and comforted family members of victims.


Bush was due to address the nation in the evening from Ellis Island, with the Statue of Liberty in the background.

Hours earlier, Bush stood in a grassy field in Shanksville, Pennsylvania, where one jet skidded to a fiery halt after its passengers rebelled against the hijackers.

The president watched silently as a red and white wreath was laid at the site of an old strip mine where Flight 93 slammed into the ground at 920km/h, killing the 40 passengers and crew on board.

Bush regularly hails Flight 93's passengers as the first heroes in the US war on terrorism, but did not speak publicly in Shanksville on Wednesday.

Instead, he exchanged private words with the assembled relatives of crash victims, asking "Are you doing okay?" and telling them "Hang in there", and "God bless you".

The war on terrorism, which began by ousting the Taleban regime in Afghanistan which harboured accused September 11 mastermind Osama bin Laden, has increasingly turned toward Iraq, despite the lack of public evidence linking that country to the attacks on America.

Speaking at the first crash site he visited on Wednesday, the Pentagon, Bush vowed to defeat what he called the merciless fanatics who used the planes as guided missiles.

"The murder of innocents cannot be explained, only endured. And though they died in tragedy, they did not die in vain," Bush said on a mild, sunny morning much like the one last year on which 184 people, excluding the five hijackers, were killed at the headquarters of the US armed forces.

The president vowed to prevail in the war on terrorism he launched after September 11.

"Today, we remember each life. We rededicate this proud symbol. And we renew our commitment to win the war that began here," Bush said.

With 13,500 people at an outdoor ceremony, Bush watched as a huge, fluttering American flag was unfurled from the newly rebuilt face of the Pentagon that was a smoking hulk of twisted metal, broken stone and shattered glass after the plane hit.

A small handwritten sign reading "Marian we miss you", barely legible through the dust still coating the new construction, was taped to the inside of a Pentagon window, a reminder of the victims in the attacks.

White House spokesman Ari Fleischer described Bush's mood as he headed to New York as "reflective and resolute". He said the day served as a "reminder of why it's so important for the president of the United States to be resolute, to see through beyond today to make sure our nation is still protected".

The president's day began with a private church service and was to end with a prime-time address to the nation against the backdrop of the Statue of Liberty. White House officials said at midday, however, that high winds in New York might force Bush to move the location of the speech from Ellis Island.

At the White House, Bush marked the anniversary with a moment of silence.

Tightly clasping the hand of his wife, Bush held the ceremony on the White House South Lawn at 8.46am (12.46am NZT), the moment the first hijacked jet hit the World Trade Centre a year ago.

Several hundred government workers formed a horseshoe as they lined the mansion's driveway, with the tall white hat of the White House chef standing out among the dark suits and strained faces of those gathered for the moment of silence.

The president started the day with a prayer service at St John's Episcopal Church across from the White House, which was surrounded by extra security officers in a reminder that the nation was on high alert for possible terrorist attacks like those that came out of a clear blue sky a year ago.

"They might have bloodied us, but they didn't break us," the Reverend Luis Leon told Bush and more than 100 worshippers, including several members of Bush's Cabinet gathered at the church to remember the victims of the September 11 attacks.

About halfway through the ceremony, a sombre-faced Bush and his wife walked to the altar of the church to light candles.

They were to have been joined by Vice President Dick Cheney and his wife Lynne but the two stayed away, with the vice president remaining at a secret location because of the fear of a new act of terrorism.

Additional officers and police cars and unmarked vehicles were deployed for several hours along Pennsylvania Avenue and around the White House complex, while elsewhere in the city anti-aircraft missile batteries were set up as an added precaution.

Fleischer said the orange alert level imposed on Wednesday, signifying a high risk of terrorist attack, would remain "so long as the intelligence indicates it should be kept at that level".