US President Barack Obama will mark the ninth anniversary of the September 11 attacks at the Pentagon, and Vice President Joe Biden will be at the World Trade Center site in New York, the White House said on Tuesday.
Officials had previously announced that First Lady Michelle Obama and former first lady Laura Bush would on Saturday mark the attacks in Pennsylvania at the site where the hijacked plane that did not reach its target crashed.
Obama also led national remembrance commemorations at the worst terror attack on US soil at the Pentagon last year, on his first September 11 as president, and called for a new national resolve in tackling Al-Qaeda.
The World Trade Center site, known as "Ground Zero," has been at the center of a fierce political row in recent weeks, over plans to build a Muslim cultural center in the vicinity, branded by critics as grossly insensitive.
Obama upset some Democratic allies last month by joining a bitter national debate over the plans, affirming the right to build on religious freedom grounds but without endorsing what he called the "wisdom" of doing so.
The Pentagon memorial is the only completed official monument to the victims of the September 11 attacks, with construction under way at similar sites in New York and Pennsylvania.
Some 2,993 people died, including 2,752 in New York, on September 11, 2001, in the attacks using hijacked planes that demolished both towers of the World Trade Center, and crashed into the Pentagon military headquarters outside Washington.