The State Department on Friday highlighted recent attacks in Saudi Arabia and Turkey and said the government "remains deeply concerned about the security of US citizens overseas".

The department issued what is called a "worldwide caution" against attacks and plane hijackings.

"We are seeing increasing indications that al-Qaida is preparing to strike US interests abroad," said the warning.

The warning came a day after two explosions on British targets in the Turkish commercial city of Istanbul killed 27 people and left another 450 wounded.

Growing fears

"We assess that other geographic locations could be venues for the next round of attacks," the State Department said.

"We are seeing increasing indications that al-Qaida is preparing to strike US interests abroad"

US State Department

"We expect al-Qaida will strive for new attacks designed to be more devastating than the September 11, possibly involving non-conventional weapons such as chemical or biological agents," it said.

The department also warned that the al-Qaida might "attempt a second catastrophic attack within the United States".

US officials say the Istanbaul attacks bore the signatures of Usama bin Ladin's al-Qaida.

Soon after the explosions, US authorities cautioned its citizens more attacks could follow in Turkey and advised them to avoid travelling to the country.