Iceland and Sweden are investigating allegations that planes flown by the CIA used their airports during secret transfers of terror suspects.
Secrecy surrounds US "war on terror" prisoner transfers
The Icelandic government says it has asked the US for an explanation and is still awaiting a satisfactory answer.
On Thursday, the Swedish government began an investigation to establish whether CIA prisoner flights had used Swedish airports.
Spain is investigating similar claims about secret flights from Majorca.
It is alleged that at least 10 CIA planes landed at airports in Majorca and the Canary Islands between January 2004 and 2005.
Icelandic media reports allege that CIA planes have landed in Iceland at least 67 times since 2001.
The CIA's controversial "extraordinary rendition" programme involves removing suspects without court approval to third party countries for interrogation.
The US state department has refused to comment on specific allegations, or to confirm or deny claims the CIA has set up secret prisons across the globe for the interrogation of "war on terror" prisoners.
UN special representative on torture Manfred Nowak called on the European Union to investigate the allegations.