US warns of fresh attacks

"At least one of these attacks could be executed by the end of the summer 2003," the department said in an advisory sent out on Saturday and obtained on Tuesday.

"Al-Qaeda planners have primarily considered suicide hijackings and bombings as the most promising method to destroy aircraft in flight as well as to strike ground targets," the advisory said.

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"Attack venues may include the United Kingdom, Italy, Australia or the East Coast of the United States due to the relatively high concentration of government, military and economic targets," according to the advisory.

"The hijackers may try to calm passengers and make them believe they were on a hostage, not suicide mission. The hijackers may attempt to use common items carried by travelers, such as cameras, as weapons," the advisory stated.

No operatives are known to have been deployed to conduct the possible attacks, it added.

US officials said the warning came from "multiple sources," including at least one captured al-Qaeda operative. But they emphasised they were still trying to determine the credibility of the information.

They said no change was anticipated in the nation's terror alert level. The level remains at "yellow" or an "elevated" risk of attack, where it has generally been since the system began in March last year.

The State Department later updated an existing "worldwide caution" to include the possibility that threats to US citizens "may also involve commercial aircraft."