* Germany wants tougher EU data rules, more disclosure from firms
* Merkel says U.S. "must stick to EU law in future"
* Surveillance threatens to become issue in September election
By Madeline Chambers
BERLIN, July 14 (Reuters) - Chancellor Angela Merkel vowed on Sunday to push for tougher EU data protection rules and force Internet firms to be more open as she tried to reassure voters before a September election about intrusive snooping by U.S. intelligence in Germany.
In an interview with ARD television, Merkel also said she expected the United States to stick to German laws in future, the closest she has come to acknowledging that its spying techniques may have breached German rules.
The question of how much Merkel and her government knew about reports of intrusive surveillance by the U.S. National Security Agency in Germany has touched a raw nerve and could yet affect the outcome of September's election.
Merkel said tighter European rules were needed.
"Germany will make clear that we want Internet firms to tell us in Europe who they are giving data to," she told ARD.
"We have a great data protection law. But if Facebook is registered in Ireland, then Irish law is valid, and therefore we need unified European rules," she said, adding that people were rightly worried about what happened to data outside Germany.
"Germany will take a strict position," she said.
Last month, U.S. officials confirmed the existence of an electronic spying operation codenamed PRISM. Former U.S. spy agency contractor Edward Snowden said it collects data from European and other users of Google, Facebook, Skype and other U.S. companies.
In a separate leak, the United States was accused of eavesdropping on EU and German offices and officials.
Merkel clearly pointed the finger at the United States.
"I expect a clear commitment from the U.S. government that in future they will stick to German law," she said.
Government snooping is a particularly sensitive subject in Germany due to the heavy surveillance of citizens practised in the communist East and under Hitler's Nazis. A magazine report last week saying German spies were colluding with the NSA caused outrage.
Merkel dispatched her interior minister to Washington last week to get answers on the spying but he has been derided by opposition parties for failing to present any U.S. assurances or concessions.
The scandal is turning into an election issue and Merkel, tipped to win a third term, needs to make sure she does not give the impression that she knew more than she has let on.
Her opposition Social Democrat rival Peer Steinbrueck accused Merkel of failing to live up to her oath.
"Mrs Merkel swore the oath of office to protect the German people from harm. Now it emerges that German citizens' basic rights were massively abused," he told the newspaper Bild am Sonntag. "I have a different view of protecting the people from harm."
Merkel said the means did not always justify the ends.
"We are fighting a war on terrorism but not everything that is technically possible ... must be carried out," she said.
She also backed calls from some of her cabinet ministers to add a protocol on data protection to an existing United Nations agreement on civil and political rights.
In the wide-ranging interview, Merkel also defended her economic record, citing low unemployment and high tax revenues, and attacked opposition plans to raise taxes on the rich.
"If you talk about raising taxes, it causes uncertainty among people and firms, and it is not necessarily the case that if you increase taxes you get higher revenues," she said.
She also said her government had been "very transparent" in spelling out the risks to voters of the euro crisis, and that a great deal had been done to stabilise the euro. (Reporting by Madeline Chambers; Editing by Kevin Liffey)