The United States has issued a new worldwide alert about the danger of possible terrorist attacks.
Attacks could be 'more devastating' than those of 11 September 2001
The US State Department said there were growing indications that al-Qaeda militants were preparing to strike American interests abroad.
The warning comes in the wake of four blasts in Istanbul over the last week which killed more than 50 people.
US authorities warn that al-Qaeda will attempt to strike in a more devastating manner than on 11 September 2001.
The latest warning suggests US interests abroad are more likely as terrorist violence increases and the Muslim holy month of Ramadan ends.
But the US Government has chosen not to raise the alert level in the US itself.
The colour-coded warning remains at amber - the midpoint on the five-level scale, indicating an elevated level of threat.
Al-Qaeda is linked to one of two groups which have claimed responsibility for the recent attacks in Turkey.
The US also accuses al-Qaeda of being behind the attacks on New York and Washington on 11 September 2001.
Worse than 9/11?
The government "remains deeply concerned about the security of US citizens overseas," the State Department warning said.
"We are seeing increasing indications that al-Qaeda is preparing to strike US interests abroad."
"We expect al-Qaeda will strive for new attacks designed to be more devastating than the September 11 attack, possibly involving non-conventional weapons such as chemical or biological agents," says the statement.
"We also cannot rule out that al-Qaeda will attempt a second catastrophic attack within the US."
A similar warning was issued by the Department of Homeland Security and the FBI, warning of "a high volume of reporting indicating possible threats against US interests during the Muslim holiday, Ramadan, and the upcoming holiday season".
This alert specifically warns of al-Qaeda's "continued interest in aviation" in carrying out attacks.
In addition to the recent bombings in Turkey, the alerts follow a suicide attack on 8 November in the Saudi Arabian capital, Riyadh, which killed 17 people.
But it appears they were also prompted by fresh intelligence, says Jon Leyne, BBC correspondent in Washington.
Britain and the US have warned their citizens of further attacks in Turkey and are both advising against non-essential travel there.