"Will there be abuses of policy? That's entirely possible," Rice said at a NATO press conference. "Just because you're a democracy it doesn't mean that you're perfect."
She offered assurances, however, that any abuses would be investigated and violators punished.
"That is the only promise we can make," Rice said. She spoke a day after trying to clarify to European foreign ministers the U.S. policy on secret prisons and treatment of terrorism suspects.
They foreign ministers appeared receptive. NATO Secretary General Jaap De Hoop Scheffer said Rice had "cleared the air."
Rice explained to the NATO and European Union foreign ministers over dinner Wednesday night that the United States does not violate the rights of terrorist suspects, De Hoop Scheffer said. "You will not see this discussion continuing" at the NATO headquarters, he told a news conference Thursday.
His comments echoed those of several foreign ministers who sought to shift away from a confrontation with Washington over the issue.
Rice repeated on Thursday that no U.S. personnel are allowed to commit abuses, whether on U.S. soil or overseas.
"The United States doesn't engage in torture, doesn't condone it, doesn't expect its employees to engage in it," she said.
Rice did not mention U.S. employees in similar statements Wednesday and earlier. Including them now may be a reference to a loophole identified by human rights organizations that could allow the CIA to contract out mistreatment.
Complaints that the United States has overstepped its bounds and may have practiced or condoned torture have dogged Rice through a four-nation tour this week. She said she welcomed a discussion of the issue, and invited it herself at the top of the private dinner with other NATO ministers.
German Foreign Minister Frank-Walter Steinmeier said Thursday that Rice "has reiterated that in the United States international obligations are not interpreted differently than in Europe."
Dutch Foreign Minister Ben Bot said he left Wednesday night's dinner "very satisfied" by Rice's comments.
Rice has refused to answer directly whether the United States keeps terrorist suspects in detention centers that violate European legal and human rights guarantees. She assured German Chancellor Angela Merkel on Tuesday that the United States would work to rectify any mistakes it has made in its war on terror.
Reports of secret prisons have roiled Europe for a month. Rice has asserted that the United States acts within the law and tried to argue that the Europeans are safer because of tough U.S. tactics.
Sustained criticism in the media and open skepticism from some European leaders indicate that the issue isn't going away.
Before Rice arrived in Brussels, Europe's leading human rights watchdog group said it hoped to be able to see satellite images of sites in Romania and Poland as part of its investigation into alleged secret CIA prisons and air transport of terrorists in Europe.
A top Council of Europe official said the body had been granted permission to look at images in the archives of the European Union's main satellite center, as well as log books held by the EU's air safety organization.
Rene van der Linden, chairman of the Parliamentary Assembly of the Council of Europe, also said America's war on terror was not a real war.
"There is ... a difference of opinion between Condoleezza Rice and the Council of Europe, because the law of war doesn't exist. We have international conventions. We are fully in favor of fight against terrorism, but we have to be aware that certain cases can not be accepted," van der Linden said.
The Strasbourg, France-based Council of Europe is the guardian of the European Convention on Human Rights, a legally binding treaty signed by all 46 council members.
The council itself has no direct jurisdiction over any country, but can exercise political pressure.