Franco Frattini, the EU justice and home affairs commissioner, spoke after Europe's leading human rights watchdog the Council of Europe voted to continue its inquiry into alleged CIA secret flights and prisons in Europe.
The commissioner said it was "a fact" that such incidents had taken place on European territory since the September 11, 2001, attacks on the US, but added the number of cases was unknown.
He also said it was not clear whether governments were aware of the renditions and had co-operated, or whether or not the renditions were legal.
"It's extremely premature to draw consequences from the elements so far available," he said.
However, Frattini called on European governments to follow up on the reports investigations.
"We must make sure that national authorities understand that they have not only the power but the duty to carry out judicial investigations.
"I am determined to encourage, to put political pressure if necessary, on the home affairs ministers so that we get results"
Franco Frattini, EU justice and home affairs commissioner
"I am determined to encourage, to put political pressure if necessary, on the home affairs ministers so that we get results (of the inquiries)," he added.
Under the CIA rendition policy, detainees are moved to third countries for interrogation, where some detainees have alleged they were tortured. Others have reportedly ended up in Guantanamo Bay.
The report, which was published earlier this month for the Council of Europe and compiled by Swiss senator Dick Marty, concluded that 14 European nations colluded with US intelligence in a "spider's web" of secret flights and detention centres that violated international law.
The US has admits picking up suspects but denies that people sent overseas have been subjected to torture. It has criticised the report as a "rehash".
Marty's report provided no direct evidence, relying mostly on flight logs provided by the EU's air traffic agency, Eurocontrol.
It also used witness statements gathered from people who said they had been abducted by US intelligence agents and judicial and parliamentary inquiries in various countries.
Human rights groups on Tuesday welcomed the report and called on European states to halt involvement in US illegal transfers or secret detentions of terror suspects and fully investigate alleged violations.
In a joint statement, Amnesty International, Human Rights Watch, the International Commission of Jurists and the Association for the Prevention of Torture said that the renditions amounted to "enforced disappearance, a crime under international law".