New York/Washington -- With a moment of silence and the dedication of a Pentagon memorial, the United States on Thursday marked the 7th anniversary of the 2001 terrorist attacks, which led the country into two wars.

US President George W. Bush called for a minute of silence at 8:46 a.m. (12:46 GMT), the moment when the first hijacked airliner smashed into the north tower of New York's World Trade Center on Sept. 11, 2001.

Within minutes, a second plane commandeered by Islamic terrorists ripped into the south tower and before the hour, a third ploughed into the Pentagon and a fourth crashed in Pennsylvania.

And afterward, 2,975 people were dead – including the 19 hijackers.

Credit for those attacks was claimed by terrorist network al-Qaeda and its leader, Osama bin Laden.

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

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

On average, 46 percent 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 Sept. 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 skeptical world and continues without NATO support.

On Thursday, the commemoration of the September 11 attacks included 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 held their ceremony of name-reading at Zucotti Park, then walked 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.

Earlier this week, New York Mayor Michael Bloomberg expressed frustration at delays in building the memorial there and insists it will be finished by the 10th anniversary in 2011.

Near Washington, US President George W Bush and Defense Secretary Robert Gates were on hand for the dedication of the Pentagon memorial -- a collection of 184 steel benches, one for each of the dead at the Defense 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/Expatica