US Secretary for Homeland Security Tom Ridge has defended the disruption caused by the cancellation and delays to some transatlantic flights.
The US says it wants to share information with allies
In an interview with the BBC, Mr Ridge said there had been "a constant stream of reports" suggesting threats to flights including those from Britain.
He also confirmed the US had been on a heightened state of alert because of fears of a radioactive "dirty bomb".
The US has been on the second highest security alert since 21 December.
Mr Ridge told the BBC's Newsnight programme that US intelligence agencies believed there was a threat of an attack during the Christmas period "equal to or greater than" the 11 September attacks.
Asked whether there had been a specific threat of the use of a biological weapon, he said a combination of specific sources and analysis had concluded that it was "a potentially intended weapon" - but not an explicit threat as such.
US scientists secretly tested radiation levels in major American cities during the holiday period, The Washington Post newspaper reported on Wednesday.
It said officials feared a "dirty bomb" could target New Year celebrations.
From 22 December, a day after the US security alert was raised to "orange", scientists were sent out to Washington, New York, Las Vegas, Los Angeles and Baltimore, the Washington Post reported.
Mr Ridge said he believed that increasing security was a deterrent.
He said flight delays between Britain and the US might be eased if the Americans were provided with more passenger information. And he conceded that the US authorities could review this information more quickly.
For a fifth day on Wednesday, British Airways flight BA223 into Washington was delayed for more than an hour as a result of increased security checks.
The same flight was cancelled on 1 and 2 January and delayed every day since then.
Mr Ridge told Newsnight there had been an indication from US intelligence agencies "that has led us to target that flight".
Tom Ridge: Better safe than sorry
But he insisted the flight was now safe and he would "absolutely" let one of his children take it.
Information on possible attacks had come from detainees - some of whom have been kept for many months without due judicial process.
Asked whether the US-led war on terror was undermining Western freedoms, Mr Ridge said: "I think that is explicitly a danger and we can't afford to let happen.
"If we undermine the basic liberties and freedoms we've enjoyed for centuries, the terrorists achieve victory."