Ten weeks before German elections, Chancellor Angela Merkel's center-left rivals have gone on the attack over reports of sweeping US online surveillance and German cooperation, sparked by fugitive intelligence analyst Edward Snowden.

There is widespread disquiet over claims, reported by news weekly Der Spiegel, that the US National Security Agency has scooped up German e-mails, online chats and phone calls and shared some of it with the country's intelligence services.

The question dividing analysts is whether the public's anger could harm Merkel politically, crack the armor of the consistently popular chancellor and damage the still-healthy poll lead of her conservative Christian Democratic Union.

If more damaging information were to come to light before the Sept 22 vote, "this could come to the boil and become a central campaign topic", said political scientist Hajo Funke of Berlin's Free University. "The debate is only just beginning."

Merkel's struggling rival from the Social Democratic Party, Peer Steinbrueck, has come out swinging, charging that Merkel broke her oath-of-office pledge to protect the German people from harm. He has called for a parliamentary inquiry.

In a newspaper commentary, he described the scale of reported US snooping as "dizzying" and blamed Merkel - saying she was either aware of it and complicit, or ignorant and therefore negligent.

Steinbrueck also poured scorn on what he called Merkel's vague comments on the scandal and her claim to have learned only from media reports of what he called the "millionfold breach of the basic rights" of German citizens.

Merkel has approached the thorny surveillance issue with characteristic caution. During a June visit by US President Barack Obama, she spoke of the need to find the right balance between security and freedom.