German Chancellor Angela Merkel has called for answers over "grave" accusations of US spying which, she said in a speech to parliament, had put transatlantic ties "to the test".
Merkel kicked off a statement to the Bundestag lower House by addressing the issue of US snooping on German soil, which included her mobile phone.
Lawmakers will later return to the chamber to debate the widespread surveillance by the US National Security Agency (NSA), which has sparked outrage in Germany and frayed ties with the US.
"The transatlantic relationship and therefore also the negotiations for a free trade agreement are presently without doubt being put to the test by the remaining accusations against the US and the collection of millions of data," Merkel said.
"The accusations are grave. They must be explained and, more important still for the future, new trust must be built," she said to applause from lawmakers.
Her remarks came at the start of an address to the Bundestag on EU partnerships with eastern European nations.
She stressed that ties with the US were of "paramount" importance for Germany and Europe.
People held demonstration in front of the Bundestag supporting former US National Security Agency contractor Edward Snowden.
After evidence of US agents having tapped Merkel's mobile phone, part of wider revelations from leaked NSA documents from whistleblower Snowden, she confronted President Barack Obama by phone last month.
Berlin also took the unusual step of summoning the US ambassador.
For months the claims have put the US in the firing line and strained diplomatic ties between Washington and its international allies, also casting a cloud over EU-US talks to clinch the world's biggest free-trade accord.