The head of the powerful US Senate intelligence committee said she was "totally opposed'' to spying on leaders of allied nations, and promised to review intelligence operations.
With mass American eavesdropping overseas roiling US-European relations, Senator Diane Feinstein stepped in to the growing row and alleged that lawmakers had been kept in the dark.
The Senate panel was reacting after reports the National Security Agency tapped Germany Chancellor Angela Merkel's phone and scooped up millions of phone records in Spain and elsewhere.
"With respect to NSA collection of intelligence on leaders of US allies - including France, Spain, Mexico and Germany - let me state unequivocally: I am totally opposed,'' Feinstein said.
"It is clear to me that certain surveillance activities have been in effect for more than a decade and that the Senate Intelligence Committee was not satisfactorily informed,'' she added.
"It is abundantly clear that a total review of all intelligence programs is necessary so that members of the Senate Intelligence Committee are fully informed as to what is actually being carried out by the intelligence community.''
Reports that US spies are tracking of millions of phone calls and emails abroad, including in allied countries, has enraged many foreign leaders and damaged the United States' public standing.
A European parliamentary delegation visited Washington today and demanded an explanation, and a separate delegation of German intelligence and government officials is expected.
The EU lawmakers met with House Intelligence Committee chairman Mike Rogers, who said he saw progress in their talks but acknowledged that more discussions and policy shifts might be needed to bridge the gaps between the two sides.
Feinstein, a member of President Barack Obama's Democratic Party who has defended the NSA against many previous allegations, did not disguise her opposition to the policies that have led to a deterioration in transatlantic ties.
"Unless the United States is engaged in hostilities against a country or there is an emergency need for this type of surveillance, I do not believe the United States should be collecting phone calls or emails of friendly presidents and prime ministers,'' she said.
Feinstein also asserted that it was a "big problem'' that Obama had not been made aware that Merkel's communications allegedly were being collected since 2002.
"The president should be required to approve any collection of this sort,'' she said.
"Congress needs to know exactly what our intelligence community is doing. To that end, the committee will initiate a major review into all intelligence collection programs.''