Germany on Sunday toughened the tone of its response to allegations that U.S. intelligence agencies spied on Chancellor Angela Merkel, warning Washington that diplomats and embassies on German soil must obey the law, which considers eavesdropping on telephone calls to be a crime.

The German government reacted to a wave of media reports that Merkel's cell phone had been targeted for more than a decade by spies working out of the U.S. Embassy in Berlin and that the operation was carried out with the apparent knowledge of U.S. President Barack Obama.

Foreign Minister Guido Westerwelle said in a statement that "German law applies on German soil," echoing a phrase uttered by Merkel last summer.

The law applies "to Germans and foreigners, to citizens and businesses, and also to diplomats and embassies," Westerwelle said.

The Bild am Sonntag newspaper reported Sunday, citing U.S. intelligence sources, that Obama was informed of the spying program by National Security Agency, or NSA, director Gen. Keith Alexander in 2010, did not ask that the program be ended and even requested a full report on the chancellor.

Obama, however, assured Merkel during a phone conversation last Wednesday that he was not aware of the spying program and would have stopped it if he had known, media reports said.

Germany plans to send a high-level delegation to the United States in the next few days to discuss the spying allegations with the White House and U.S. intelligence agencies.

The NSA eavesdropped on calls made with the phone provided to Merkel by her political party until July and even intercepted calls made on the secure phone that the German leader started using last summer, Bild am Sonntag reported.

In July, the chancellor swapped her Nokia for a government-issued encrypted Blackberry Z10.

The phone calls were intercepted by the 18 NSA employees working on the fourth floor of the U.S. Embassy in Berlin and reports were sent directly to the White House without first processing them at the spy agency's headquarters in Fort Meade, Maryland.

The weekly Der Spiegel reported that Merkel's cell phone has been on an NSA target list since 2002, three years before she took office. EFE