BERLIN - Former German Chancellor Gerhard Schroeder accused the United States of showing "no respect" for Germany's sovereignty after a newspaper reported on Wednesday the U.S. National Security Agency (NSA) had bugged his phone from at least 2002.

Reports last year about mass U.S. surveillance in Germany, in particular of Chancellor Angela Merkel's mobile phone, shocked Germans and sparked the most serious row between the transatlantic allies in a decade.

"The United States has no respect for a loyal ally and for the sovereignty of our country," Schroeder was quoted as saying by Bild daily. "That countries spy on each other is certainly not a new experience. But to spy on the telephone of a chancellor is clearly a step too far."

Since the first reports of spying last year, it has been widely suspected in Germany that the NSA had bugged governments preceding Merkel's. But the report by Sueddeutsche daily on Wednesday, which it sourced to former NSA contractor Edward Snowden, was the first concrete report that offers evidence.

Sueddeutsche also cited U.S. government sources and NSA insiders as saying the reason for the snooping on Social Democrat (SPD) Schroeder was his opposition to the U.S.-led invasion of Iraq under then President George W. Bush.

"We had reason to believe that (Schroeder) was not contributing to the success of the alliance," the newspaper quoted one person with knowledge of the monitoring as saying.

Schroeder, who headed a coalition with the Greens between 1998 and 2005, said the United States should have respected Germany's opposition to the Iraq invasion, after it had shown "a high degree of solidarity" with respect to Afghanistan.

Merkel said last week that Berlin and Washington were still "far apart" in their views on the NSA's surveillance of Germany but that they remained close allies. Berlin has been pushing, so far in vain, for a 'no-spy' deal with Washington.

"Even if it's not easy with the Americans, we must continue to push for an international agreement," German Justice Minister Heiko Maas was quoted by Spiegel Online as saying.

"We should not leave any stone unturned in trying to protect better the data of people in Germany."

Maas said the NSA used the pretext of security to collect data unchecked. "Listening to chancellors' mobile phones does not help, at any rate, to protect from terror attacks."