The fallout since Merkel spoke with Obama to protest National Security Agency eavesdropping led to recriminations between allies and calls to suspend U.S.-European Union trade talks. Merkel reacted to an internal probe showing that her mobile phone could have come under U.S. surveillance.
Spain's Foreign Ministry summoned U.S. Ambassador James Costos to respond to a report that the NSA monitored millions of phone calls by Spaniards, according to an emailed statement. The NSA's work helps protect U.S. allies and their interests, as well as U.S. citizens, Costos said.
"The U.S. and Spain have enjoyed a long friendship based on our shared values and a history of cooperating to advance mutual interests around the globe," Costos said in a separate statement. "We will continue to work closely with Spain on a wide range of issues, including the collective security of our two countries and of U.S. and Spanish citizens."
Obama's spokesman, Jay Carney, refused to comment on any details of U.S. surveillance programs or what other leaders may have been targets of NSA snooping. With a trans-Atlantic trade pact high on Obama's agenda, Carney said the U.S. doesn't use such surveillance to advance American economic interests.
"We do not use our intelligence capabilities for that purpose," he said Monday in Washington. "We use it for security purposes."
German authorities are in the process of clarifying the circumstances of the possible surveillance of Merkel's phone, government spokesman Steffen Seibert told reporters Monday.
"If this information is confirmed, then this would represent a grave breach of trust," Seibert said. Still, "where trust has been lost, it has to be restored."
A group of high-level German intelligence officials is traveling to Washington to discuss the matter, he said.
The Obama administration denied a Bild report over the weekend that the president had been informed by NSA Director Keith Alexander about tapping Merkel's phone three years ago and allowed the operation to continue. Bild cited an unidentified high-level official within the NSA.
"General Alexander did not discuss with President Obama in 2010 an alleged foreign intelligence operation involving German Chancellor Merkel, nor has he ever discussed alleged operations involving Chancellor Merkel," the NSA said in a statement distributed by the White House. "News reports claiming otherwise are not true."
Obama told Merkel he was unaware of the surveillance as he apologized to the chancellor in their Oct. 23 conversation, Der Spiegel magazine reported, citing unidentified chancellery officials.
U.S. authorities obtained Merkel's cell number in 2002, Spiegel reported, citing documents including those disclosed by former government contractor Edward Snowden. The surveillance is being carried out by an NSA "Special Collection Service" from within the U.S. Embassy adjacent to Berlin's Brandenburg Gate, Spiegel cited the documents as showing.
The NSA shut down the Merkel operation and others targeting world leaders after the administration conducted a probe over the summer, the Wall Street Journal reported, citing unnamed U.S. officials. Obama had been briefed on "intelligence priorities," though not specific operations, the Journal said.
Carney refused to comment directly on the report.
The SPD's Nahles said that Snowden, the source of the surveillance disclosures who is in temporary asylum in Russia, could be called on to give testimony to a probe by the Bundestag, parliament's lower house, according to Bild. The SPD is currently negotiating to form a coalition government with Merkel's Christian Democratic bloc.
Some lawmakers said the U.S. is justified in conducting a sweeping spying program.
Obama should "stop apologizing" for NSA surveillance that has "saved thousands of lives," Rep. Peter King, R-N.Y., said Sunday on NBC's "Meet the Press" program.
With assistance from Ben Sills in Madrid.