Amnesty International has accused the United States of using front companies to transfer individuals to countries where they have faced torture or ill-treatment.
The fresh charges come after months of allegations by campaigners who say the Central Intelligence Agency transports terrorism suspects outside normal legal channels to countries where they could be tortured under interrogation.
Washington says it does sometimes transfer suspects outside normal extradition procedures - a practice known as rendition - but denies sending them to countries that use torture.
Amnesty said in a report it has records of nearly 1,000 flights directly linked to the CIA, mostly using European airspace, which were made by planes that appear to have been permanently operated by the CIA through front companies.
Amnesty International secretary general Irene Khan
It also said it had records of about 600 other flights made by planes confirmed as having been used at least temporarily by the CIA.
"The latest evidence shows how the US administration is manipulating commercial arrangements in order to be able to transfer people in violation of international law," said Amnesty International secretary general Irene Khan in a statement.
"It demonstrates the length to which the US government will go to conceal these abductions."
Critics say it is difficult to prove what such flights were used for and point out that flights used by the CIA may have been simply carrying officials.
Amnesty said it has linked the aircraft to people who have been illegally transferred. It cites one plane known to have made over 100 stops at Guantanamo Bay.
Another took suspect Hassan Mustafa Osama Nasr, also known as Abu Omar, to Egypt from Germany after he was caught in Italy, said the group.
Amnesty called on the aviation sector to take action to ensure companies do not lease their aircraft in circumstances where they may be used in renditions.