* Row over U.S. surveillance refuses to die down in Germany
* Minister's fact-finding trip to U.S. much criticized
* Big questions remain over who knew what in Germany
BERLIN, July 16 (Reuters) - German Chancellor Angela Merkel faced pressure on Tuesday to toughen her stance against the United States over its spy programme, after her interior minister failed to convince lawmakers he had received answers during a trip to Washington.
Hans-Peter Friedrich briefed a confidential parliamentary committee in charge of intelligence issues about his talks in the United States last week, saying these were just the start of a long process of discussion and investigation.
But the spying, and questions over how much Berlin and its own security agencies knew about it, has touched a raw nerve in Germany, where it has become an election issue, as the opposition, lagging badly in the polls, tries to thwart Merkel's campaign for re-election to a third term in office in September.
"The German foreign intelligence agency (BND) and the government continue to dispute they had any knowledge of the PRISM programme. I personally consider this to be highly unlikely, given the close cooperation between the U.S. and German intelligence agencies," said opposition Social Democrat (SPD) lawmaker Thomas Oppermann, who chairs the committee.
"The chancellor has to put more pressure on (Washington) now for an explanation - we need hard facts," he said after the briefing.
Last month, the United States confirmed the existence of an electronic spying operation codenamed PRISM after ex-spy agency contractor Edward Snowden revealed that it mines data from European and other users of Google, Facebook, Skype and other U.S. companies.
In a separate leak, Washington was accused of eavesdropping on EU and German offices and officials.
Merkel, whose tone has become sharper in recent days, said during a speech in Cologne: "I want to say to our American partners: German law always applies on German soil and we will enforce it."
Germany's Bild newspaper reported on Monday that the BND had known about U.S. surveillance and storage of German data for years and used it in cases of Germans kidnapped abroad.
Merkel, who has said she first learned about the U.S. surveillance programme from the media, has pledged to seek tougher European Union data protection rules.
A poll last week by the Forsa opinion research institute showed 4 out of 5 Germans did not believe the government when it said it knew nothing of NSA spying.
Friedrich told German television earlier on Tuesday that nobody knew what PRISM was and on what scale the United States was storing information - opening himself to criticism he had been fobbed off by the Americans.
Wolfgang Neskovic, an independent lawmaker and intelligence expect called Friedrich's trip a "pure public relations gesture", but added the opposition's criticism was hypocritical - saying they had behaved exactly the same during their most recent spell in government.