The US Senate's intelligence committee has announced a major review of the country's surveillance operations.
The committee's chair, Diane Feinstein, said eavesdropping on leaders of friendly nations was wrong.
She said the White House had told her such surveillance would stop, but a senior administration official told the BBC there was no policy change so far.
Senior US intelligence agency officials are to testify before the House of Representatives later on Tuesday.
Correspondents say pressure is growing on the White House to explain why President Barack Obama apparently did not know about the extent of the intelligence gathering operations.
Mr Obama has spoken publicly of his intent to probe spying activities amid claims of eavesdropping on US allies.
In a US TV interview, the US president said that national security operations were being reassessed to make sure the National Security Agency's (NSA) growing technical spying capability was kept under control.
"We give them policy direction," he told ABC's Fusion network.
"But what we've seen over the last several years is their capacities continue to develop and expand, and that's why I'm initiating now, a review to make sure that what they're able to do, doesn't necessarily mean what they should be doing."
An EU delegate in Washington has described the row over intelligence gathering as "a breakdown of trust".
German media has reported that the US bugged German Chancellor Angela Merkel's phone for more than a decade - and that the surveillance only ended a few months ago.
Ms Feinstein, who chairs the Senate intelligence panel, called for a "total review" of US intelligence programmes in light of the Merkel revelations.
"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," she said in a statement.
"It is my understanding that President Obama was not aware Chancellor Merkel's communications were being collected since 2002. That is a big problem."
Senator Feinstein said the White House had told her that all surveillance of leaders of countries friendly to the US would stop.
However, the BBC's North America editor Mark Mardell says a senior administration official has told the BBC this is not accurate - and that while there have been individual changes - there have not been policy changes, such as terminating intelligence gathering aimed at allies.
Earlier, White House spokesman Jay Carney told reporters that the administration "recognise[s] there needs to be additional constraints on how we gather and use intelligence".
Neither Mr Carney nor Mr Obama have commented on specific allegations that the US eavesdropped on international allies, including tapping the phones of foreign officials.
An across-the-board review of US intelligence resources is currently under way.
The head of the NSA, Gen Keith Alexander, and other experts are due to testify before the House of Representatives intelligence committee at 13:30 local time (17:30 GMT) on Tuesday.
The BBC's Jonny Dymond in Washington says tough questions can be expected from Congress as politicians of all stripes have been angered by the revelations of large scale intelligence gathering on both Americans and US allies.
The Associated Press quoted an unnamed administration official saying the Obama government was considering ending spying on allied heads of state.
The official said a final decision had still to be made, as the internal review was under way. (BBC)
