Angela Merkel's main rival in the German general election next month has pledged to suspend EU-US trade talks over the National Security Agency spying scandal if he replaces her as chancellor.

In a further sign that the NSA row has soured relations between the US and its European allies, Peer Steinbrück, leader of the Social Democratic party, told ARD TV: "I would interrupt the negotiations until the Americans say if German government offices and European institutions are bugged or wiretapped. We don't know if the Americans may be sitting under our desks with some technical devices." He pledged to press Washington about the spy agency's activities before continuing with the talks about a transatlantic free trade agreement.

His move comes after Germany's Der Spiegel weekly reported that the NSA bugged the United Nations' New York headquarters, with the European Union and the International Atomic Energy Agency among those targeted.

Der Spiegel said files obtained by whistleblower Edward Snowden showed that the NSA spied on the EU in New York after it moved to new rooms in autumn 2012 and that the NSA runs a bugging programme in more than 80 embassies and consulates worldwide called the "Special Collection Service", which has "little or nothing to do with warding off terrorists".

Other files cited in the report describe how the NSA managed to access the UN's video conferencing system and crack its security.

Snowden's revelations about the NSA have plunged privacy-conscious Germany into outrage during the campaign for the election on 22 September. Distrust of the government's handling of the surveillance scandal threatens to damage Merkel's bid to retain the chancellorship.

Earlier this month, her government attempted to reassure voters that US and British intelligence agencies had observed German laws in Germany. But critics remained sceptical, pointing out that would not stop foreign agencies accessing German online communication data transferred to the US-based servers of Google, Facebook or Microsoft.

At a recent rally, the chancellor struggled to make herself heard over chants of "hypocrite" and "liar" and the sound of vuvuzelas being blown by opponents worried about stories of American snooping.

Germany's independent privacy watchdogs say the surveillance programmes run by the NSA breach an EU-US pact meant to ensure cross-border data protection. "We're just at the beginning of the debate," Frank-Walter Steinmeier of the Social Democrats said on Friday. "The assurances ... do not refer to data taken from American servers."

• The picture caption on this article was amended on 26 August 2013. An earlier version incorrectly described Peer Steinbrück as the German finance minister.