Air travellers from the United States and Britain have been alerted to the possibility of attack if they go to Saudi Arabia.
There were warnings of threats to flights out of Riyadh
The alerts came after intelligence reports were relayed to Washington and London by the Saudis of a specific threat to British airlines flying in or out of Riyadh international airport.
British Airways, one of the world's biggest airlines, suspended all flights to Saudi Arabia until further notice.
Later, the US State Department renewed an existing warning to US citizens to avoid non-essential travel to the kingdom.
Gun battles between the security forces and suspected Islamic militants in the kingdom have left several people dead over the past few days.
The Americans are particularly sensitive to threats in Saudi Arabia following the suicide car bombings in May on residential blocks housing Americans and other foreigners.
In May 2002, a shoulder-fired missile only just missed a US military transporter taking off from a military airbase in northern Saudi Arabia.
The BBC's David Bamford, in Washington, says the latest reports contribute to a heightened sense of unease about flying in general, as Americans prepare for the second anniversary of the attacks on the World Trade Centre and the Pentagon.
BA's decision followed advice from the UK Department for Transport which warned of "credible intelligence of a serious threat to UK aviation interests in [Saudi Arabia]".
The source of the scare lies in the Saudi capital Riyadh, where the authorities have been trying to break up Islamist terror cells, said BBC security correspondent Frank Gardner.
"By watching Riyadh airport they've learned this week that al-Qaeda sympathisers also had the airport under surveillance," said our correspondent.
BA usually operates four flights a week each to the Saudi cities of Riyadh and Jeddah.
Other airlines, including Saudi Arabian Airlines which flies direct to Riyadh, and several other Middle Eastern airlines which have indirect routes, are still flying to Saudi.
The Saudi ambassador to Britain, Prince Turki al-Faisal, said Saudi Airlines had offered to fly BA passengers to and from the kingdom.
"The threat is specifically to British aviation," he added.
'All means necessary'
Passengers at Heathrow, some of whom were on their way to Mecca, were dismayed to find out that their flights were cancelled - and many heard through the assembled media rather than the airline.
The Foreign Office said it would alert the British community in Saudi Arabia to the announcement, via its warden network.
It said it did not intend to change its current advice about travel to Saudi Arabia. This already advises against all but essential travel to the Middle East country.
It states: "If you choose to travel to, or remain in, Saudi Arabia you should take all necessary steps to protect your safety and should make sure you have confidence in your security arrangements."
Bomb attacks in Riyadh on 12 May this year killed more than 30 people, including two British nationals, and injured nearly 100.
Passengers seeking information on BA flights to Saudi Arabia can ring the airline's main number on: 0870 8509850.