New intelligence shows British Airways and Air France flights to the US could be terrorist targets, say US officials.
The Air France flight to Los Angeles was grounded in December
The intelligence is said to specifically mention BA flight 223, which was cancelled twice in early January amid heightened attack fears.
The US Homeland Security Department said all flights remained unchanged at present, and there were no plans to raise the current alert level.
US officials are talking to British and French authorities on the next steps.
The BBC's Michael Buchanan in Washington said US intelligence agencies reported receiving a "specific and credible threat" in the last couple of days.
It is feared al-Qaeda may be attempting to hijack a plane from either London or Paris and fly it into a target in the United States, added our correspondent.
"We remain concerned about al-Qaeda's desire to target aviation, especially international aviation," said Brian Roehrkasse of the Homeland Security department.
"The US intelligence community continues to gather specific credible threat information on international flights, as we have done in an ongoing basis in the past few weeks.
"We have shared this information with our international partners, and will work with them to put in place the appropriate security measures".
The intelligence is said to specifically mention British Airways 223 from London to Washington DC. Two such flights were cancelled and others delayed in early January.
In December, six Air France flights to Los Angeles were cancelled.
It came as the United States raised its national terror threat level to orange following a "substantial increase" in intelligence suggesting attacks using hijacked planes.
An Air France spokeswoman in New York said there was good co-operation between the airline and US and French government officials over the latest intelligence.
"We have been complying with all security measures," said Diane Cornman.
A BA spokesman said it would never choose to operate a flight unless it was completely safe to do so.