US to 'review' spying activities amid European outrage

"We give them policy direction," he said.

"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," Obama said.

It is the second review he has ordered this year, following the publication of NSA documents leaked by Snowden.

His comments come after Germany, France and Spain summoned the respective US ambassadors in their countries over revelations that the NSA engages in mass surveillance and has tapped Chancellor Angela Merkel's phone.

A delegation of seven MEPs has also travelled to Washington this week to seek explanations about the spying scandal.

"It is not acceptable, for example, this espionage on Chancellor Merkel and others since more than 10 years," said German centre-right MEP Elmar Brok, who chairs the foreign affairs committee in the European Parliament and is member of the delegation to Washington.

Brok told AFP that eavesdropping on the German administration "is in breach of German law."

Meanwhile, the US Senate is also ordering a "major review" of spy operations.

Senator Dianne Feinstein, a member of Obama's Democratic Party who chairs the intelligence committee in the upper chamber, said the Senate was kept in the dark by the NSA.

"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.

In her view, it would be a "big problem" if Obama had not been made informed that Merkel's calls were being monitored, saying "the president should be required to approve any collection of this sort."

"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," Feinstein said.

The heads of US intelligence services, including the NSA, will appear before her committee on Tuesday and testify in a public hearing.