New York, Washington - With a moment of silence and the dedication of a Pentagon memorial, the United States on Thursday marks the seventh anniversary of the 2001 terrorist attacks, which led the country into two wars.

US President George W Bush has called for a minute's silence at 8:46 am (1246 GMT), the moment when the first hijacked airliner smashed into the north tower of New York's World Trade Center on September 11, 2001.

Within minutes, a second plane commandeered by Islamic terrorists ripped into the south tower. By hour's end, a third plane ploughed into the Pentagon in Washington and a fourth crashed in Pennsylvania, apparently after a struggle for control between passengers and hijackers.

Within little more than two hours, 2,975 people were dead.

Some were crushed and incinerated in the twin infernos. Three hundred New York firefighters and police officers died, trapped as they tried to rescue people from the horror of the collapsing towers. Still other victims worked at the Pentagon or were passengers on the planes.

All 19 terrorist suspects died.

Credit for the 9/11 attacks was claimed by terrorist network al- Qaeda and its leader, Osama bin Laden, in Afghanistan.

According to an international poll released Wednesday, there is still broad scepticism that it was al-Qaeda. Majorities in only nine of 17 countries said they believed that al-Qaeda was behind the attacks, the Maryland-based WorldPublicOpinion. org reported.

A total of 22 per cent believed there was a conspiracy by the US government or Israel.

On average, 46 per cent of the 16,000 people who were questioned believed al-Qaeda was really behind the attacks.

Steven Kull, director of the polling agency, said it was "remarkable" that seven years later, there was no international consensus "given the extraordinary impact the 9/11 attacks have had on world affairs."

He was referring to the invasions of Afghanistan and Iraq, which both now have democratically elected governments.

The October 2001 US-led invasion of Afghanistan ousted the militant Islamist Taliban regime, which had protected al-Qaeda, but the battle, now under NATO forces, continues against a stubborn and renewed insurgency.

The March 2003 US-led invasion of Iraq found less support from a sceptical world and continues without NATO support.

On Thursday, the commemoration of the September 11 attacks will include flags flown at half mast on public buildings and private homes around the country.

Presidential nominees Democrat Barack Obama, 44, and Republican John McCain, 72, agreed to drop their campaign rhetoric for the day and meet at Columbia University in New York for a memorial service.

In New York, families of the victims will start their ceremony of name-reading at Zucotti Park, then walk to nearby Ground Zero, where the World Trade Center once stood, and where they will be allowed to descend a ramp to the lowest level available.

New York Mayor Michael Bloomberg expressed frustration Monday at delays in building the memorial there, and insisted it be finished by the 10th anniversary in 2011.

In Washington, US President George W Bush and Defence Secretary Robert Gates will be on hand for the dedication of the Pentagon memorial - a collection of 184 steel benches, one for each of the dead at the Defence Department headquarters.

The 59 benches dedicated to the passengers on board American Airlines Flight 77 are positioned so that visitors must face the sky when reading the individual engraved names.

The remaining 125 benches are positioned to turn visitors toward the Pentagon, commemorating the military and civilian workers inside the building when Flight 77 struck. (dpa)