Washington: The United States raised its alert level at military bases ahead of the 10th anniversary of the 9/11 attacks, as a report warned of "gaps and weaknesses" in the nation's security apparatus.

The Pentagon said it had upped security at installations as "a prudent and precautionary measure," and not in response to a particular threat, as attention began to build towards Sunday's commemorations.

A decade on, America is marking 9/11 with a series of events to honor the nearly 3,000 people who died when hijackers crashed planes into the World Trade Center, the Pentagon, and a field in rural Pennsylvania.

The biggest occasion will be the reading of victims' names at Ground Zero, an event that will be attended by victims' family members, President Barack Obama, former president George W Bush, and New York Mayor Michael Bloomberg.

Announcing the decision to increase security, a Pentagon official said al-Qaeda had "focused on holidays and milestone events in the past," noting that the 10-year anniversary "was mentioned in the documents seized" in the US raid that killed Osama bin Laden at his compound in Pakistan on May 2.

The al-Qaeda leader was reportedly planning a major attack to coincide with the 10-year anniversary of September 11, but US officials have stressed there was no evidence the "nascent plot" ever went beyond the planning phase.

Security is the main concern among organizers of all ceremonies remembering those who died and large crowds are expected as the country reflects on a decade of war abroad and fear of further attacks at home.