Hillary Clinton said there had been a "specific, credible but unconfirmed report" of an attempt to target New York and Washington DC.

President Barack Obama is to visit New York and Washington on September 11 as the US marks 10 years since al-Qaeda attacked.

Nearly 3,000 people died in New York, Washington and Pennsylvania in 2001.

Speaking after she attended 9/11 commemorations at the New York Stock Exchange, Mrs Clinton said the threat - which is reported to involve bomb attacks using truck or cars - was the work of the radical group that attacked the US 10 years ago.

"Al-Qaeda again is seeking to harm Americans and in particular to target New York and Washington," Mrs Clinton said on September 9.

"This should not surprise any of us. It is a continuing reminder of the stakes in our struggle against violent extremism, no matter who propagates it, no matter where it comes from, no matter who its targets might be."