German newspapers Bild am Sonntag quoted US intelligence sources as saying that US National Security Agency (NSA) chief Keith Alexander briefed Obama on the operation involving Merkel in 2010 but the president did not halt the operation.
The daily also said the NSA targeted not only Merkel's business mobile phone but also her official telephone device.
The German government has voiced its anger at possible US intelligence's spying on Merkel's mobile phone communications, saying it would be "a serious breach of trust".
Germany: US spying on Merkel's phone a 'serious breach of trust.'
High-ranking representatives of German security services as well as chancellery will travel to Washington next week to seek clarifications of widespread US spying allegations, government spokesman Georg Streiter said on Friday.
The German representatives are due to hold talks with White House officials as well as representatives of the NSA, the spokesman added.
According to German media reports, the European Union will also send representatives to the US to talk about the alleged US surveillance activities in Europe.