The German government voiced its anger on Wednesday evening at possible US intelligence's spying on the mobile phone communications of German Chancellor Angela Merkel, saying it would be "a serious breach of trust" if confirmed.

In an unusual sharply-worded statement, Merkel's spokesman Steffen Seibert said that Merkel called US President Barack Obama on Wednesday after her government had received information on the possible spying.

"She made it clear that she unequivocally disapproves such practices should they be confirmed and deems them as completely unacceptable," the statement said.

"Among close friends and partners, as Germany and the US have been for decades, there should be no such monitoring of communications of a head of government," Seibert said, adding that "This would be a serious breach of trust."

Merkel has also called on Washington to clarify the extent of possible surveillance activities in Germany and to give answers to "questions that the German government asked months ago," Seibert said.

The White House has immediately denied that the US tapped Merkel's phone calls.

"The United States is not monitoring and will not monitor the communications of the chancellor," news reports reaching here quoted White House spokesman Jay Carney as saying on Wednesday.

Citing classified documents disclosed by fugitive former US National Security Agency contractor Edward Snowden, German magazine Der Spiegel reported in June that Germany is high on the list of telephone telecommunication and Internet message tapping, as half a billion phone calls, emails and Internet chat messages could be intercepted by US intelligence monthly on average.

Merkel had voiced concerns over the possible electronic surveillance when Obama visited Germany in June.

In immediate reaction to the latest revelations concerning Merkel's mobile phone, German news media said that the German-USrelations is apparently facing a harsh stress test.

"The unusual announcement of the government and the sharp word choice underscores how explosive the matter is," said the online edition of the German daily Frankfurter Allgemeine.

Another German daily Die Welt said that the German Federal Office for Information Security (BSI) has been examining clues for days to the possible UStapping of Merkel's mobile phone.