The European Union is to formally ask the US to clarify reports that it ran secret CIA prisons in eastern Europe.
Spain said the allegations if true could damage ties with the US
The US has refused to confirm or deny the reports, which surfaced in the US earlier this month.
A European investigator is seeking satellite images of Romania and Poland, alleged sites of the secret prisons.
Spain, Sweden and Iceland are looking into separate reports that CIA planes stopped in their territory while transporting terror suspects.
The European investigator, Swiss Senator Dick Marty, is looking into what he called the suspicious movement patterns of flights in the region.
"This is absolutely not a crusade against America," he said.
"I think all Europeans agree with Americans that we must fight terrorism.... but this fight has to be fought by legal means," the Associated Press quoted him as saying.
"Wrongdoing only gives ammunition to both the terrorists and their sympathisers."
The UK Foreign Office has confirmed that Britain will be writing to the US, on behalf the EU, to clarify the reports of secret prisons, which were reportedly set up after the 11 September 2001 attacks.
Denials and investigations
The Washington Post newspaper first reported on 2 November that the CIA had been using Soviet-era camps in eastern Europe to detain and interrogate terror suspects.
It did not name the countries, but a day later Human Rights Watch said it had evidence indicating the CIA transported terror suspects captured in Afghanistan to Poland and Romania.
Poland and Romania have denied the allegations.
Last week, 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 while Iceland says it has asked the US for an explanation and is still awaiting a satisfactory answer.
The CIA's controversial "extraordinary rendition" programme involves removing suspects without court approval to third party countries for interrogation.