Secretary of State Condoleezza Rice, in a rare concession to US critics, acknowledged on Tuesday that Washington may make mistakes in its battle against terrorism and promised to put them right if they happened.
But she restated her defence of the legality of US tactics against a militant enemy which "operates from within our society and is intent... on killing innocent civilians".
"Any policy will sometimes result in errors, and when it happens, we will do everything we can to rectify it," Rice said at the start of a European tour overshadowed by allegations of illegal CIA methods against terrorist suspects.
Speaking in Berlin, she declined to comment on the case of a German man, Khaled el-Masri, who was allegedly abducted to Afghanistan and imprisoned there for five months last year until the CIA realised it had got the wrong man.
But German Chancellor Angela Merkel said the US government had acknowledged it blundered over Masri, who plans to sue the CIA in a case to be filed in the United States later on Tuesday.
"I'm pleased to say that we spoke about the individual case, which was accepted by the United States as a mistake, and so I'm very pleased the foreign minister (Rice) has reiterated that if mistakes are made, they must immediately be rectified," Merkel told a joint news conference.
Rice was not challenged directly over reports the United States had run secret prisons to hold terrorism suspects in eastern Europe, which Washington has refused to confirm or deny. But she defended US methods in its struggle against 21st century militants.
"If you don't get to them before they commit their crimes, they will commit mass murder," she said. "We have an obligation to defend our people and we will use every lawful means to do so."
She stressed that the United States operated strictly "within the context of laws and our international obligations".
In comments over the past two days, Rice has sought to shift from defence to attack over US tactics, countering criticism by telling Europeans that US intelligence also prevents attacks and saves lives in their own countries. There were signs she had succeeded in drawing some of the heat out of the issue in her talks with Merkel, who welcomed her assurances.