A statement purportedly from al-Qaeda militants in Saudi Arabia has warned of new attacks on US and Western airlines and other transport facilities.
Saudi Arabia is has problems with domestic Islamist militants
"All compounds, bases and means of transport, especially Western and American airlines, will be a direct target," it said.
Posted on an pro-al-Qaeda website, it asks Muslims to keep away from Westerners and such locations.
The statement's authenticity could not be immediately verified.
But it was signed "al-Qaeda on the Arabian Peninsula".
Attacks on a residential compound in Khobar at the end of May brought to the fore the major domestic problem Saudi Arabia faces with Islamist militants
Twenty-two civilians were killed - including an American, a Briton and an Italian - during the attack which saw Saudi commandos storm the compound.
The latest warning also comes as Saudi security forces hunt gunmen who attacked a BBC news team in a drive-by shooting in Riyadh on Sunday.
The specific threat to airlines will alarm an industry that has put a renewed emphasis on counter-terrorism security since the 11 September suicide hijackings in the US.
'All guns blazing'
Another bloody attack in Saudi Arabia would mean another massive blow to international confidence in the oil-rich Gulf kingdom.
Thousands of foreigners from Asia and the west work in the Saudi oil industry.
But Prince Turki al-Faisal, Saudi ambassador to Britain, on Monday defended the way his country was dealing with the threat from al-Qaeda, in an interview with the BBC.
Saudi Arabia was taking the threat from al-Qaeda seriously, but some of the country's operations against the group had been successful, and "some not so successful".
But he said Saudi Arabia was not going to "go in all guns blazing" and arrest thousands of people.
"That is precisely what the terrorists want us to do - to antagonise the population," he said.
"There is a more methodical way of doing things - police work, investigations, co-operating with other agencies and other countries," he added.