US marks seventh 9/11 anniversary

New York has paused to remember the moments two planes hit the World Trade Center on 11 September 2001 - an attack that killed nearly 3,000 people.

Four minutes of silence are being held to mark the times when four hijacked passenger planes hit the Twin Towers, the Pentagon and a Pennsylvania field.

Presidential candidates Barack Obama and John McCain are attending a ceremony at Ground Zero in New York.

At the Pentagon, President George W Bush will dedicate a new memorial.

The memorial in Washington was built at a cost of $22m (£12.6m) on a 1.9-acre (0.77-hectare) parcel of land within view of the crash site.

New York City Mayor Michael Bloomberg opened Thursday's memorial event, where families of the victims paid tribute to and read out a rollcall of the names of those who died.

Mr Bush stood for a moment of silence with First Lady Laura Bush on the South Lawn of the White House at the time the first of the two passenger planes hit the World Trade Center.

It is the last time Mr Bush marks the anniversary as president.

The attacks are regarded as the defining moment of his time in office so far, and they had a huge impact on the foreign policy of his administration.

"The president thinks about 9/11 every single day when he wakes up and before he goes to bed," White House press secretary Dana Perino said on Wednesday.

'Put aside politics'

Senators Obama and McCain, the Democratic and Republican nominees in November's election, will appear together at Ground Zero in the afternoon to lay wreathes in honour of the victims.

In a joint statement from the campaigns announcing their decision to visit Ground Zero together, the two men vowed to come together "as Americans" and suspend their political campaigns for 24 hours.

"All of us came together on 9/11 - not as Democrats or Republicans - but as Americans," the statement said. "In smoke-filled corridors and on the steps of the Capitol; at blood banks and at vigils - we were united as one American family.

"We will put aside politics and come together to renew that unity, to honour the memory of each and every American who died, and to grieve with the families and friends who lost loved ones," it said.

Their appearance is to be followed by another in the evening at a Columbia University forum to discuss their views on public service.

The ceremony in downtown Manhattan will mark the times when the planes hit the Twin Towers, and the times when each tower fell - pausing for silence at 0846, 0903, 0959 and 1029.

Family members and students representing the 90 countries that lost people in the attacks will also read out the names of all the 2,973 dead.

Seven years after the attacks which shocked the world, Ground Zero is a construction site.

After years of delays and disagreements over how to commemorate the dead, work has finally begun on a memorial and a new skyscraper - the Freedom Tower - which is due to be completed by 2012.

On Wednesday, Mr Bloomberg called for the abolition of the WTC planning agency, saying the reconstruction was "frustratingly slow, owing in large part to a multilayered governance structure that has undermined accountability from the get-go".

"Most important, the memorial must be completed by the 10th anniversary. No more excuses, no more delays," he added.

New York State Governor David Paterson said he also shared "a sense of disappointment and frustration at the unacceptable pace of the Ground Zero rebuilding".

On the eve of the anniversary, a top US military commander warned that new tactics were needed to win the conflict in Afghanistan, which the US and its allies invaded weeks after 9/11.

They intended to topple the Taleban regime and root out Osama Bin Laden, who the US believes masterminded the attacks.

Admiral Mike Mullen believes insurgents are launching attacks from neighbouring Pakistan, and US-led forces must target their "safe havens" in that country.

"In my view, these two nations are inextricably linked in a common insurgency that crosses the border between them," he said.

Pakistan has refused to allow foreign troops to fight on to its territory.