The US government may have been monitoring German chancellor Angela Merkel's cell phone for more than 10 years, according to a report by Der Spiegel based on internal documents from the National Security Agency (NSA).

President Barack Obama told Merkel that he was not aware her phone was being bugged, or he would have stopped it. However, he declined to say whether her phone had been monitored in the past. A separate report in Die Welt said the number of Merkel's Nokia 6120 Slide was listed in leaked NSA documents, although she's since switched to a BlackBerry Z10 smart phone.

The White House assured Merkel that her phone is not being tapped now. But today's report suggests that the surveillance went back as far as 2002, when Merkel was the head of her political party, the Christian Democratic Union. She became chancellor in 2005.

It's unclear how much data was being collected. Der Spiegel reports the surveillance was being done from a "not legally-registered" intelligence office. The agency reportedly has 80 such offices around the world, including Paris, Madrid, Rome, Prague, Geneva, and Frankfurt, according to a 2010 NSA document.

European leaders are growing increasingly incensed

European leaders are growing increasingly incensed over new revelations of NSA spying on foreign citizens and governments. Details are still being revealed by former federal contractor-turned-whistleblower Edward Snowden, who turned over an unknown number of internal NSA documents to journalists.

Germany and France are now pushing the United Nations to pass a measure to protect their citizens' privacy against foreign spying, and Germany is sending representatives to Washington.

Correction: An earlier version of this post said the US intelligence office in Berlin was legally registered; in fact an internal document reportedly said it was a "not legally registered spying branch" in the US embassy in Berlin.