NSA director Keith Alexander had briefed President Obama on the mobile phone tapping against Merkel in 2010, German newspaper Bild am Sonntag reported on Sunday.

"Obama did not halt the operation but rather let it continue," according to the paper citing a high-ranking NSA official.

The newspaper also said the US spying agency eavesdropped on Merkel's predecessor, Gerhard Schroder, after former US president George W. Bush launched the spying program in 2002, adding that Schroder's refusal to support the Iraq war was a key reason behind the operation.

The new revelation comes one day after German magazine Der Spiegel said that the NSA's Special Collection Service (SCS) had listed Merkel's mobile telephone since 2002.

Washington’s ally has demanded explanations from the White House after disclosures about the huge and broad American electronic spying.

On Thursday, Germany summoned US ambassador John Emerson to discuss the tapping allegations.

"For us, spying on close friends and partners is totally unacceptable. This undermines trust and this can harm our friendship," German Foreign Minister Guido Westerwelle said. "We need the truth now."

In a phone call with Obama on Wednesday, Merkel said that she "unequivocally disapproves of such practices and sees them as completely unacceptable. There should be no such monitoring of the communication of a head of government. That would be a grave breach of trust."

The White House, however, rejected the allegations, saying “the United States is not monitoring and will not monitor the communications of Chancellor Merkel."

The global outrage over US government surveillance further spiked after The Guardian -- citing a confidential memo obtained from American whistleblower Edward Snowden - revealed that the NSA is illegally eavesdropping on phone conversations of 35 world leaders.

The revelations have prompted Brazil and Germany to begin drafting a UN General Assembly resolution to restrain the NSA’s surveillance programs against other nations.

SHI/SHI