Spain's foreign minister has said there is no evidence that landings by US planes on Spanish soil were illegal.
Spain has promised closer checks on foreign planes making stopovers
It follows media reports that planes operated by the US secret service, the CIA, had made secret stops on Spanish soil while carrying terror suspects.
Foreign Minister Miguel Angel Moratinos said Spain's government was convinced US stopovers were legal but promised closer checks on aircraft in future.
The US has refused to confirm or deny reports of secret CIA jails in Europe.
Mr Moratinos said Spain was among the first European governments to seek an explanation from the US on the matter.
Washington had responded that "there was no violation of Spanish laws", he said.
Last week, Spain's Interior Minister Jose Antonio Alonso announced an investigation into a series of alleged landings in Majorca and in the Canary Islands.
The European Union said on Tuesday it would formally ask the US to clarify reports that it ran secret CIA prisons in eastern Europe.
Reporting to parliament, Mr Moratinos said the government would "strengthen the supervision of civil aircraft".
He said the Spanish government was convinced that all the US stopovers took place "within the framework of the law".
Opposition politicians have accused their government of a cover-up.
The CIA's controversial "extraordinary rendition" programme involves removing suspects without court approval to third-party countries for interrogation.
According to Spanish press reports, the CIA is suspected of having used Majorca for such prisoner transfers on at least 10 occasions.
The flight destinations from Majorca allegedly included Libya, Algeria, Romania, Macedonia and Sweden, Spanish media reported.
Relations with the US are already under considerable strain, says the BBC's Danny Wood in Madrid.