A senior EU official has warned that any member state found to have secret CIA detention centres on their soil could face serious sanctions.
Franco Frattini, European Commissioner for Justice and Home Affairs, said no evidence had proven allegations that the US intelligence agency has been hiding al-Qaeda captives at facilities in eastern Europe, as reported in the Washington Postearlier this month.
"Were these events to have occurred, then clearly this would constitute a grave infringement of the values and rules of the European Union," he told lawmakers at the European Parliament.
"Such a serious breach, where it is proven, may lead to serious political sanctions being taken against a member state of the European Union."
Human Rights Watch has identified Romania and EU member Poland as two countries possibly operating such jails. Romania is a candidate to join the EU.
Both have told the European Commission that there are no CIA jails on their territory.
EU lawmakers pressed Mr Frattini to start a more formal, in-depth investigation into the allegations, but he said such an inquiry went beyond the commission's powers.
The Council of Europe, the continent's top human rights watchdog with no institutional relationship to the EU, opened an investigation into the issue earlier this month.