Spain is launching an investigation into claims that CIA planes carrying terror suspects made secret stopovers on Spanish soil.
Secrecy surrounds US "war on terror" prisoner transfers
Interior Minister Jose Antonio Alonso made the announcement on Spanish television on Tuesday.
He said that if proven, such activities could damage relations between the Spanish and US governments.
According to Spanish press reports, the CIA is suspected of having used Majorca for such prisoner transfers.
"If it were confirmed as true, we would, of course, be looking at very serious cases," Mr Alonso told the private channel Telecinco.
The suspect flights - 10 in total - came to light in a report submitted by Spain's Civil Guard to the prosecutor's office of the Balearics Supreme Court in June, Spain's El Pais newspaper reported.
The first flight allegedly landed in Palma, on the island of Majorca, on 22 January 2004.
The suspect flights - by two Boeing 747s and two Gulfstream jets - allegedly continued until 17 January 2005.
Meanwhile, members of the European Parliament have urged the European Commission to investigate claims that the CIA used prisons in eastern Europe for the interrogation of terror suspects.
Spain's Defence Minister Jose Bono reacted cautiously to the Majorca allegations on Tuesday, saying "we do not have any evidence, we do not have any proof".
He denied a report that the Spanish secret service had asked the CIA to stop using the airport at Palma.
The flight destinations from Majorca allegedly included Libya, Algeria, Romania, Macedonia and Sweden, Spanish media reported.
Spain's relations with the US cooled when Prime Minister Jose Luis Rodriguez Zapatero withdrew Spain's contingent of troops from Iraq shortly after taking office in March 2004.
The Popular Party of Jose Maria Aznar, who had backed the US-led war in Iraq, was ousted in the election, just days after the 11 March train bombings in Madrid, which left 191 people dead.