An investigation into claims that the Central Intelligence Agency ran secret prisons to detain and interrogate suspects rounded up in America"s "war against terror" is to be launched by the Polish government.

The Prime Minister of Poland, Kazimierz Marcin-kiewicz, announced the inquiry yesterday, just as the US Secretary of State, Condoleezza Rice, returned to Washington after a tour of European countries in which she attempted to defuse the furore over the clandestine prison claims.

Poland is one of a number of Eastern European countries where, it is claimed, terror suspects have been flown for intensive interrogation. The US administration will now come under even greater pressure to provide details of its programmes over recent years to round up suspects allegedly tied to al-Qa"ida. The US is believed to be detaining up to two dozen such people in hidden locations.

"I am commissioning a detailed check in all places possible to precisely check if there is any proof that such an event took place in our country," Mr Marcinkiewicz told Polish television. "It is necessary to finally close the issue because it could be dangerous to Poland."

On Friday, it appeared that European foreign ministers had agreed to accept assurances from Ms Rice, laid out during a Nato meeting in Brussels, that the US does not condone the use of "cruel, inhumane and degrading" treatment on detainees whether in the US or abroad.

It was unclear if that amounted to a policy shift by the US. Previously, Washington has indicated that the ban on such treatment did not apply to Americans interrogated overseas. Practices attributed to CIA investigators include "water-boarding", where suspects are strapped to planks, the heads covered in plastic and dunked in water.

Earlier in a tour that also took in Germany, Romania and Ukraine, Ms Rice also tried to make the case that Washington was obliged to take vigorous steps to capture terrorists and gain information for them and that European governments should support that strategy.

But public outrage, sparked by reports of prisons, the clandestine use of European airports and airspace by unmarked CIA planes and the practice of "rendition", where suspects are delivered to foreign regimes not squeamish about torture, may not have been much assuaged by Ms Rice"s diplomacy.

The decision of Poland, a staunch US ally, to look deeper into the prison claims is embarrassing for Washington. Warsaw may have had little choice, given the appearance in a Polish newspaper of comments by New York-based Human Rights Watch, asserting that the US had been using Polish territory to hold and question suspects and that some had been transferred there via Romania.

"Poland was the main base of interrogating prisoners and Romania was more of a hub," the organisation"s analyst, Marc Garlasco, was quoted as saying in the newspaper Gazeta Wyborcza. "This is what our sources from the CIA tell us and what is shown from the documents we gathered."