The threat is unconfirmed but reportedly involves Washington DC and New York, where people are preparing to mark the 10-year anniversary of 9/11, the worst terrorist attack on US soil, in which nearly 3,000 people died.
New York Mayor Michael Bloomberg said the new threat was uncorroborated, but he asked citizens to report suspicious activity and said that security would be increased at bridges, tunnels and on public transport.
He added: “For the record, I plan to take the subway tomorrow morning.”
9/11 anniversary
The anniversary of 9/11 will be marked by a ceremony at Ground Zero, attended by President Barack Obama as well as former president George W Bush.
The White House said President Obama had been briefed on the threat against the United States.
It noted that the US government has “enhanced its security posture” ahead of the anniversary. White House spokesman Jay Carney added: “We’re hyper-vigilant to this specific report that’s just coming in.”
Speaking at a news conference, the FBI assistant special agent in charge of the New York office, Janet Fedarcyk, confirmed the terror threat.
She said: “Al-Qaeda has shown an interest in important dates and anniversaries. In this instance it is accurate that there is credible, specific but unconfirmed information.”
Documents discovered in Osama bin Laden’s compound in Abbottabad, Pakistan, after he was killed in a raid in May by US Navy Seals, highlighted his persistent interest in attacking the US around the anniversary of the 2001 attacks. But it is unclear if the plans ever evolved beyond aspiration.