US Secretary of State Condoleezza Rice has signed a deal allowing the US to use military bases in Romania.
The deal marks a major shift in US strategy in Europe
It is the first such deal to be signed with a former communist country in eastern Europe.
Ms Rice arrived in Romania from Germany, on the second leg of a European tour overshadowed by a controversy about CIA operations.
She stressed that the US does not carry out or condone torture, but did not comment on alleged secret CIA prisons.
She has defended the CIA practice of "renditions" - secretly transferring terror suspects to third countries.
There is widespread concern across Europe about reports of secret CIA "rendition" flights from European countries and claims that the CIA used some prisons in eastern Europe outside international law.
War on terror
American civil rights campaigners are taking legal action against the US government over the case of a Lebanese-born German national - Khaled al-Masri - allegedly abducted by the CIA.
German Chancellor Angela Merkel told a joint news conference that democratic rules should be adhered to, but added that intelligence services must be able to do their job.
Ms Rice said the US would fulfil its "international obligations" and would seek to rectify any mistakes made.
Romania is one of the countries that human rights activists believe hosted a CIA secret prison - a charge the Romanian government denies.
"It was never the case of Romania hosting any facilities like prisons," Romania's Foreign Minister Mihai-Razvan Ungureanu told the BBC.
He signed the bases deal with Ms Rice, who praised Romania as one of Washington's "best allies".
Under the bases agreement, the US military will have access to Mihail Kogalniceanu air base in south-east Romania, just inland from the Black Sea.
It is part of the planned redeployment of US forces in Europe, the BBC's defence and security correspondent Rob Watson reports.
The plan, first announced last year by the Pentagon, involves both the reduction and repositioning of American troops in Europe.
Some bases are being closed in western Europe, with new ones to be opened in the east.
It is a strategic shift intended to place US forces closer to potential areas of conflict in North Africa, the Middle East and Central Asia, our correspondent says.
Romania hopes the basing agreement will bring both diplomatic and economic gains, he says.
Romania assured the US on Tuesday that it would not withdraw its contingent of 860 troops from Iraq.
Ms Rice will later travel to Kiev and Brussels.