Senior US spy chiefs are scheduled to appear before the House intelligence committee amid reports the US has eavesdropped on leaders of US allies.
Director of National Intelligence James Clapper and National Security Agency head Gen Keith Alexander will testify.
Earlier the US Senate’s intelligence committee announced a major review of the country’s surveillance operations.
President Barack Obama faces criticism over reports he was unaware of the extent of the spying.
Tuesday’s hearing follows calls by US Senate intelligence committee chairwoman Diane Feinstein to end eavesdropping on leaders of friendly nations.
President Obama says he wants to ”review” the NSA’s operations
Ms Feinstein 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.
In a television interview, the US president said the country’s national security operations were being reassessed to ensure the National Security Agency’s (NSA) growing technological 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.”
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. It has also been reported that the NSA spied on French diplomats in Washington and at the UN, that it conducted surveillance on millions of French and Spanish telephone calls, among other operations against US allies.
‘Totally opposed’
In the wake of the Merkel revelations, Ms Feinstein called for a “total review” of US intelligence programmes.
Jay Carney: “There has been extraordinary change… in the way we transmit and gather information”
“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.”
Ms Feinstein said the White House had told her that all surveillance of leaders of countries friendly to the US would stop.
However, a senior administration official has told the BBC that 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.
Also on Monday, 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”.
The BBC’s Jonny Dymond in Washington says Gen Alexander and Mr Clapper can expect tough questions from Congress as politicians of all stripes have been angered by the revelations of large-scale intelligence gathering on both Americans and US allies.
Spying pact
What do people in Spain make of news 60 million calls were tracked?
Earlier on Monday, representatives from the European Parliament’s Committee on Civil Liberties, Justice and Home Affairs spoke to members of the US Congress about the alleged US spying on European leaders and citizens.
The European delegation was reportedly unhappy with the “stock” responses from US officials on the issue.
Their visit coincided with reports that the US had monitored 60 million Spanish telephone calls in a month and asked the Japanese government to help it monitor fibre-optic cables carrying personal data through Japan, to the Asia-Pacific region.
According to Spain’s El Mundo newspaper, the NSA tracked tens of millions of phone calls, texts and emails of Spanish citizens in December 2012 and January 2013.
The Japanese news agency Kyodo said Japan refused the NSA’s request, citing legal restriction and staff shortages.
Mrs Merkel is also sending German intelligence officials to Washington.
The allegations of US surveillance on international allies stem from documents leaked by fugitive ex-US intelligence contractor Edward Snowden, now living in Russia.
The US has had a “no-spying pact”, known as Five Eyes, with Britain since just after World War II, with Australia, New Zealand and Canada later joining.(BBC)