U.S. President Barack Obama has met with his top national security advisers to review measures to safeguard commemorations for the 10th anniversary of the September 11, 2001 terrorist attacks.

No details of that meeting were made public. But it came as authorities remained on high alert following an intelligence tip that al-Qaida may have sent three men to the U.S. to set off a car bomb in New York or Washington around the anniversary.

Tight security was already in place across the nation as the anniversary approached, and police bolstered their presence in New York and Washington after the tip was received several days ago.

Officials said the threat of a possible attack would not disrupt ceremonies scheduled for Sunday — which is the anniversary itself.

The White House also says the threat will not change the president's plans. Mr. Obama is scheduled Sunday to visit the sites of the attacks in New York City, as well as at the Pentagon and in Shanksville, Pennsylvania.

Authorities shut down part of Dulles International Airport near Washington Saturday because of a suspicious object but later said nothing harmful was found.