Washington  - The hard-fought presidential campaign was put on ice Thursday out of respect for the seventh anniversary of the 2001 terrorist attacks.

But the nominees were participating in some of the formal commemoration activities at the sites where 19 Islamic terrorists crashed hijacked passenger planes into the symbols of America's financial and military might on that September day.

John McCain, 72, the Republican presidential nominee, spoke 90 seconds at the crash site of Flight 93 in Shanksville, Pennsylvania, where passengers attempted to regain control of the plane to prevent it from being flown into the US Capitol, which some believe was its intended destination.

In a 90-second commentary, McCain saluted those on board for their courage, saying they may have saved hundreds of people on Capitol Hill and possibly even himself. Forty passengers died in the crash.

They "deprived hateful enemies of their triumph," McCain said.

McCain and Democratic nominee Barack Obama, 44, agreed earlier this week to suspend campaigning and campaign advertising on Thursday.

The two men are to make a joint appearance later Thursday at Ground Zero in New York, where 2,751 people were killed at the World Trade Center after two hijacked planes crashed there.

They were also to join a discussion at Columbia University labelled as a "presidential forum" on service.

In a statement, Obama noted the "spirit of service" that came after the terrorist attacks, in terms of donations of blood and money, and asked for a renewal of that spirit.

He saluted the "heroic efforts of our firefighters, police and emergency responders, and those who sacrificed their own lives on Flight 93 to protect their fellow Americans."

He also gave "thanks for the Americans defending our security in our communities at home, and in our military abroad."

The fourth plane crashed into the Pentagon in Washington, DC, killing 184 people in the plane and on the ground. A memorial was dedicated there Thursday by US President George W Bush - the first memorial to be finished.

Memorials in Shanksville and in New York Ground Zero are to be finished by the tenth anniversary, in 2011. (dpa)