Spokesman Steffen Seibert said Merkel made clear in her call that "she views such practices, if the indications are confirmed ... as completely unacceptable" and called for US authorities to clarify the extent of surveillance in Germany.

A statement from Seibert said the German government "has received information that the chancellor's cellphone may be monitored by American intelligence".

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

The White House said Obama has assured Merkel that the US is not listening in on her phone calls.

White House spokesman Jay Carney said the two leaders spoke by phone, adding: "The United States is not monitoring and will not monitor the communications of the Chancellor."

Carney said the US is examining Germany's concerns as part of a review of how the US gathers intelligence.

The White House has cited that review in responding to similar spying concerns from France and other US allies.

Snowden, a former NSA worker, went public with revelations about US spying operations in June.

The information he leaked led to claims of systematic spying by the NSA and CIA on a global scale.

Targets included rivals like China and Russia, as well as allies like the EU and Brazil.

The NSA was also forced to admit it had captured email and phone data from millions of Americans.

NTJ/NJF