Chancellor Angela Merkel's mobile phone may have been monitored by the NSA, a German government spokesman has said.

Ms Merkel spoke to President Barack Obama over the phone on Wednesday to seek clarification after Germany said it had "received information that the chancellor's cell phone may be monitored by American intelligence".

The Chancellor's spokesman, Steffen Seibert said the government leader told Mr Obama that "she unequivocally disapproves of such practices, should they be confirmed, and regards them as completely unacceptable".

"Between close friends and partners, as Germany and the US have been for decades, there should not be such monitoring of the communications of a government leader," he said.

"This would be a grave breach of trust. Such practices should be immediately stopped."

The government would not elaborate on the claims but news magazine Der Spiegel, which has published material from National Security Agency leaker Edward Snowden, said its research triggered the response.

The White House issued a statement saying the president assured Ms Merkel that the US was not monitoring her communications.

"The United States greatly values our close co-operation with Germany on a broad range of shared security challenges," it said.

"As the President has said, the United States is reviewing the way that we gather intelligence to ensure that we properly balance the security concerns of our citizens and allies with the privacy concerns that all people share.

"Both leaders agreed to intensify further the co-operation between our intelligence services with the goal of protecting the security of both countries and of our partners, as well as protecting the privacy of our citizens."

The German allegations emerged just days after France summoned the US ambassador to explain a report in Le Monde newspaper saying the NSA had swept up 70.3 million French phone records during a 30-day period.

Mr Obama also told President Francois Hollande that the US was reviewing its intelligence-gathering to ensure a balance between security and privacy.

The White House has said recent disclosures about the NSA have been distorted, but acknowledged some reports raised "legitimate" questions for America's allies about how its intelligence capabilities are used.

Mexico and Brazil have also complained to the US about apparent spying by American agencies.