Airlines in the United States have been warned that the al-Qaeda terror network could carry out hijackings or suicide bombings by the end of the summer.
Four planes were hijacked in the 11 September attacks
The US Department of Homeland Security, set up after the 11 September attacks in 2001, also warned that US interests in countries such as the UK, Italy and Australia could be targeted.
The alert sent out to US airlines and law enforcement agencies warns that al-Qaeda has considered suicide hijackings and bombings as the best way to destroy planes as well as strike ground targets.
The BBC's Ian Pannell, in Washington, said the warning was in stark and unusually blunt language.
"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.
"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."
Australian officials have since claimed that the country was not on an al-Qaeda list of potential targets and say the US will clarify the position.
Nineteen hijackers were involved in the hijack of four planes for the 11 September attacks which resulted in the death of thousands of people when two aircraft were flown into the World Trade Center.
Since then, al-Qaeda is believed to have been responsible for a number of terror attacks around the world including bombings in Saudi Arabia and Morocco.
The warning added that the suicide-hijack method could be used again.
"The hijackers may try to calm passengers and make them believe they were on a hostage, not suicide mission," it says.
"The hijackers may attempt to use common items carried by travellers, such as cameras, as weapons," the advisory stated.
Last week a 900-page congressional report was published criticising intelligence failings ahead of the 11 September attacks.
Among key points highlighted by the report were that agencies missed opportunities to disrupt the plot by detaining would-be hijackers and that inadequate attention was given to the threat of a terrorist attack against the US.