German Chancellor Angela Merkel called Monday for answers over "grave" accusations of U.S. 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 U.S. snooping on German soil, which included her mobile phone. Lawmakers will later return to the chamber to debate the widespread surveillance by the U.S. National Security Agency (NSA), which has sparked outrage in privacy-sensitive Germany and frayed ties with the United States. "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 U.S. 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 U.S. were of "paramount" importance for Germany and Europe. After evidence of U.S. agents having tapped Merkel's mobile phone, part of wider revelations from leaked NSA documents from U.S. fugitive Edward Snowden, she confronted President Barack Obama by phone last month. Berlin also took the unusual step of summoning the U.S. ambassador. For months the claims have put the U.S. in the firing line and strained diplomatic ties between Washington and its international allies, also casting a cloud over EU-U.S. talks to clinch the world's biggest free-trade accord.