The United States has acknowledged that it needs to make changes to its intelligence gathering procedures.
The pressure is on the US to regain the trust of its European allies, amid a growing number of allegations that it has been spying on EU leaders and citizens.
The White House said that President Barack Obama had ordered a review of US surveillance practices.
White House Press Secretary Jay Carney told reporters: “We recognise that there need to be additional constraints on how we gather and use intelligence.”
A group of European Members of Parliament (MEPs) have been holding talks with members of Congress in Washington on the issue.
British Labour Party MEP Claude Moraes said he and his fellow delegates were not getting all the answers they needed:
“Trust has to be rebuilt. We need to figure out why this kind of mass surveillance activity is happening and what kind of trust needs to be rebuilt,” Moraes said.
The spying allegations, which have been fuelled by leaks to the press from NSA whistleblower Edward Snowden, have soured US-EU relations.
The latest revelations included claims that the US had listened in on 60 million Spanish phone calls a month.