The Washington Post said on Wednesday the CIA had been hiding and interrogating inmates at a secret Soviet-era facility in Eastern Europe.
The detention facility is one of several so-called "black sites" that formed a global network set up after the 11 September 2001 attacks on New York and Washington.
"We are concerned at the fate of an unknown number of people captured as part of the so-called global war on terror and held at undisclosed places of detention," Antonella Notari, chief ICRC spokeswoman, said.
"Access to detainees is an important humanitarian priority for the ICRC and a logical continuation of our current work in Afghanistan, Iraq and Guantanamo Bay," she added.
Also in Geneva, the United Nations' Human Rights Committee said it had received two letters and a report from the US which it hoped would address the issue of detainees being held outside the country.
"Access to detainees is an important humanitarian priority for the ICRC"
Chief ICRC spokeswoman
"It was in our request to the United States. We are going to see how they answer," committee chairwoman Christine Chanet for France told journalists, saying the committee had yet to study the documents.
The European Commission said on Thursday it would look into media reports naming two east European countries as allowing the CIA (Central Intelligence Agency) to hold al-Qaida suspects outside of any national or international legal jurisdiction.
Friso Roscam Abbing, spokesman for European Justice and Security Commissioner Franco Frattini, said the EU executive would check the reports with Poland, a new member state, and Romania, which is due to join the European Union in 2007.
Human Rights Watch
The US-based campaign organisation Human Rights Watch said earlier it had indications the two were hosting CIA prisons.
Romania and Poland denied the allegations on Thursday and Abbing said he had no knowledge of any such prisons at present.
The CIA is accused of hiding
inmates at so-called black sites
"What I think we will do is to, at a technical level, ... check what the truth is in these stories. We will check the accuracy of those reports," he told a daily briefing.
He said the treatment of prisoners was not a matter of EU competence but any secret prisons would not appear compatible with the EU's non-binding Charter of Fundamental Rights or the so-called Copenhagen political criteria for EU membership.
He said the commission's decision to check the reports did not signal any formal investigation, nor would it be appropriate for Frattini to personally question government leaders in the countries concerned.
Carroll Bogert, associate director of the New York-based Human Rights Watch, outlined earlier what had led the group to believe Poland and Romania were hosting the alleged CIA prisons.
She said the group based its assumption on flight logs, such as a Boeing 737 having made trips to eastern Europe from Afghanistan and countries in the Middle East.
One flight log showed that a plane went from Kabul to northeastern Poland on 22 September 2003. That was the same month that "we know several CIA prisoners who were held in Afghanistan were transferred out of Afghanistan and the next day the same plane landed at a military airport in Romania," Bogert said.
The Washington Post said it had not published the names of the European countries at the request of senior US officials who said disclosure could disrupt counter-terrorism efforts or make the host countries targets for retaliation.
In reaction to that report, Czech Interior Minister Frantisek Bublan was quoted by the online news outlet Aktualne.cz as saying that the Czech Republic recently turned down a US request to set up a detention centre on its territory.
"The negotiations took place around a month ago," he was quoted as saying, adding that they did not succeed.
US declines comment
US officials declined direct comment on the report, which was likely to stir up fresh criticism of the Bush administration's treatment of terrorism suspects.
Russia's FSB security service and Bulgaria's foreign ministry both denied such facilities existed on their territory as did Thailand, which was named in The Washington Post report.
The UN's Human Rights Committee monitors a 1976 treaty on basic freedoms. The regular report on compliance filed by the US last Friday was some seven years overdue.
The committee, which will examine the report next July at a public session, said last year it had specifically asked that the issue of detention centres be included.
The ICRC, a neutral humanitarian organisation, monitors whether prison conditions and treatment of detainees comply with the 1949 Geneva Conventions on the treatment of prisoners of war.