EU leaders say the distrust of the US over spying could harm the fight against terrorism, after new leaks revealed that Washington has routinely monitored the phone conversations of 35 world leaders.
Allegations of spying overshadowed proceedings at an EU summit in Brussels which was supposed to focus on Europe's growing refugee crisis.
"[European leaders] stressed that intelligence gathering is a vital element in the fight against terrorism. This applies to relations between European countries as well as to relations with the USA,” Herman van Rompuy, the president of the European Council, said in remarks on Friday.
"A lack of trust could prejudice the necessary cooperation in the field of intelligence gathering," he said.
The British newspaper The Guardian said on Thursday it had obtained a confidential memo suggesting the NSA was able to monitor 35 world leaders' communications in 2006.
The memo said the NSA encouraged senior officials at the White House, Pentagon and other agencies to share their contacts so the spy agency could add foreign leaders' phone numbers to its surveillance systems, the report said.
The Guardian did not identify who reportedly was eavesdropped on.
What is at stake is preserving our relations with the United States.
A delegation of members of the European Parliament will travel to Washington on Monday for three days to seek a response to the allegations.
The nine-member delegation from the parliament’s civil liberties committee will meet senior US government and intelligence officials and explore "possible legal remedies for EU citizens" resulting from the alleged surveillance, although it is not clear what such remedies might entail.
The allegations have prompted German Chancellor Angela Merkel to say that the reports have shattered trust in the US administration and undermined the crucial trans-Atlantic relationship.
"It's become clear that for the future, something must change, and significantly," Merkel said.
"We will put all efforts into forging a joint understanding by the end of the year for the cooperation of the [intelligence] agencies between Germany and the US, and France and the US, to create a framework for the cooperation."
France, which also vocally objected to allies spying on each other, asked that the issue of reinforcing Europeans' privacy in the digital age be added to the agenda of the two-day summit.
"What is at stake is preserving our relations with the United States," French President Francois Hollande told reporters at his own early-morning news conference. "They should not be changed because of what has happened. But trust has to be restored and reinforced."
Swedish Prime Minister Fredrik Reinfeldt called it "completely unacceptable" for a country to eavesdrop on an allied leader, and Dutch Prime Minister Mark Rutte called the spy allegations "exceptionally serious".
Echoing Merkel, Austria's foreign minister, Michael Spindelegger, said, "We need to re-establish with the US a relationship of trust, which has certainly suffered from this."