Members of the European Parliament have urged the EU's executive Commission to investigate allegations that the CIA used secret prisons in eastern Europe.
Human rights groups have criticised US treatment of prisoners
But EU Justice Commissioner Franco Frattini told the MEPs late on Monday that it was up to individual governments to investigate the claims.
Recent reports in the US press alleged that US secret agents had interrogated terror suspects in eastern Europe.
Poland and Romania - named in the allegations - have denied the reports.
Mr Frattini said that if proven, such actions would be "a serious infringement of the principles of the European Union," which could lead to political sanctions against a member state.
Poland joined the EU in May 2004 and Romania is a candidate for membership.
But Mr Frattini insisted that he did not have the power to launch an official inquiry - a response that one MEP, British Liberal Democrat Baroness Ludford, described as "feeble".
"Can we seize classified files of the CIA? No, that's not possible. We don't have the powers, we have to play it by the rules," Mr Frattini said.
Baroness Ludford told BBC Radio 4's Today programme that the claims threatened to damage the EU's credibility as an upholder of human rights.
"I'm not impugning the good faith of the Polish and Romanian governments, but we do know that secret intelligence services sometimes have their own agenda.
"I'm left with a sense of unease and residual doubt when respected organisations have made allegations but we cannot honestly say they've been thoroughly investigated," she said.
"I think our whole mechanisms in the EU for ensuring human right standards are respected are frankly inadequate."
The claims have been denied by all sides, but the US Senate has demanded a report from its intelligence chief.
In Italy, a separate investigation has already led to a court ordering the arrest of 21 suspected CIA agents accused of helping to kidnap a Muslim cleric in 2003.
There has also been concern in Spain about the alleged use of an airport on the island of Mallorca by so-called CIA "plane prisons".
The reports by a local Spanish newspaper, Diario de Mallorca, prompted an official inquiry, the Associated Press news agency says.
According to the Washington Post newspaper, the CIA has had jails in Europe and Asia. The centres - known as "black sites" - were allegedly set up in the wake of the 11 September 2001 attacks on the US.
About 30 detainees, considered major terrorism suspects, were held by the CIA at the "black sites", the paper said.
At least 70 other detainees have since been handed over to intelligence services in Egypt, Jordan, Morocco, Afghanistan and other countries, the paper said.
The Washington Post named Afghanistan and Thailand as hosts of secret jails, which are now said to have closed. Thailand has issued a denial.