A US official has said claims that the CIA illegally used Europe's airspace to transfer terror suspects are damaging US-EU co-operation on intelligence.
A report by MEPs has said the CIA ran 1,000 flights within the EU
John Bellinger, a legal adviser to the US secretary of state, said there had been only a few such flights in Europe.
Last month, European lawmakers cited a thousand or more cases of such flights.
Washington's critics say the practice amounts to the seizure of suspects by CIA agents for interrogation, including torture. The US denies the accusations.
"The suggestion that intelligence flights are somehow engaged in illegal activity really undermines the co-operation between the United States and Europe," Mr Bellinger told reporters in Brussels.
He was responding to a European Parliament inquiry which concluded that the CIA had run more than 1,000 flights within the European Union since 2001, often transporting terror suspects for questioning overseas.
Mr Bellinger said that most flights were only carrying officials or forensic evidence, and suggestions that many had detainees on board were "absurd".
"Someone needs to challenge that," he said. "It's not possible for the United States to prove a negative, but responsible European governments or responsible European officials simply need to say this has gotten out of hand."
He also defended the use of the camp at Guantanamo Bay, Cuba, where hundreds of terrorism suspects have been held for up to four years without trial.
However, he admitted the US had perhaps not been clear enough in explaining the legal basis for Guantanamo.
He refused to comment on individual rendition cases, or on interrogation tactics such as sleep deprivation or simulated drownings.
But he said the US did not use torture or get others to do it on Washington's behalf.