BERLIN  — German opposition parties accused Chancellor Angela Merkel's interior minister on Saturday of failing to nail down the extent of U.S. spying on allies such as Germany, a sensitive issue that could influence a September election.

The question of how much the government knew about reports of intrusive surveillance by the U.S. National Security Agency (NSA) has touched a raw nerve in Germany just two months before the election, in which Merkel is tipped to win a third term.

Interior Minister Hans-Peter Friedrich met U.S. Vice President Joe Biden and other officials in Washington on Thursday and Friday in an attempt to find out more about reports of widespread U.S. monitoring in Germany and Europe.

Opposition parties derided Friedrich who after his talks defended U.S. surveillance of global communications, saying it had foiled 45 attacks in the world, including five in Germany.

"The same minister who four weeks ago said there was no data scandal has been fobbed off (by Washington)," Peer Steinbrueck, running against Merkel for the opposition Social Democrat party (SPD), told Bild am Sonntag weekly.

"Either this minister is naive to the limit and thus incompetent, or his understanding of our constitution is alarming to say the least," he said.

Government snooping is a particularly sensitive subject in Germany due to heavy surveillance in the communist East and under Hitler's Nazis. A magazine report last week saying German spies were "in bed with" the NSA caused an outcry.

After his talks, Friedrich also said Berlin would not tolerate any breach of German law by the NSA and that the two countries would look at scrapping a decades-old pact allowing joint intelligence operations to protect U.S. troops in Germany.

"The trip was a disaster, Friedrich has come back empty-handed," said senior SPD figure Thomas Oppermann. "We still don't know anything about the millions of surveillance (operations) in Germany."

Polls give Merkel's conservatives a lead of up to 19 points over the SPD but it is unclear if she will be able to continue her center-right coalition with the Free Democrats (FDP), who are struggling even to win enough votes to enter parliament