US Secretary of State Condoleezza Rice has admitted that terror suspects are flown abroad for interrogation, but denied they were tortured.
Germany is one of the countries reported as a landing spot
She said suspects were moved by plane under a process known as rendition, and that this was "a lawful weapon".
She refused to address claims that the CIA runs secret prisons abroad where suspects are interrogated without reference to international law.
She then flew to Europe, where she can expect tough questions on the issue.
The allegations have caused an uproar, and the EU has written to Washington asking for clarification.
Ms Rice's first stop is Germany, where parliament has demanded to know the purpose of more than 400 flights, run by the US military, that landed or passed through German airspace.
She will also visit Romania - where human rights groups allege a detention centre may have been located - and Brussels.
On Monday, human rights group Amnesty International said that six planes used by the CIA for renditions had made 800 flights in EU airspace, including 50 landings at Shannon airport in the Republic of Ireland.
Ms Rice told Ireland last week that Shannon airport had not been used for "anything untoward".
Ms Rice made a robust defence of US policy of transporting suspects to another country for interrogation.
She said the US would use "every lawful weapon to defeat these terrorists", who were often essentially stateless and did not fit into any traditional military or criminal justice system.
"We have had to adapt," she declared.
She said the US always respected the sovereignty of allies, abided by the law and did not allow torture.
The BBC's Clive Myrie in Washington says Ms Rice's comments reflected indignation that anyone would accuse the US of condoning torture.
Her tour of Europe will probably see her pressing her hosts for more co-operation, and less criticism, in the "war on terror", our correspondent says.
Ms Rice refused to address the question of secret prisons directly.
"We cannot discuss information that would compromise the success of intelligence, law enforcement, and military operations," she said in a statement at Andrews Air Force Base, Maryland.
The secretary of state said European countries often benefited from, and even assisted, US intelligence-gathering.
She said the governments involved could decide for themselves whether they wanted to disclose information about any CIA-run prison.
She said renditions had been carried out for decades between the US and its allies.
"Renditions take terrorists out of action, and save lives," she said. "Such renditions are permissible under international law."
However, some legal experts have suggested that the process of rendition is open to challenge under international law.