Westerwelle and members of a US congressional delegation visiting Berlin and Brussels said they aimed to emerge from the crisis with stronger security and economic ties but that it would take concrete steps.
"We are working together so that trust can be reestablished," Westerwelle said after talks with Senator Chris Murphy and Representative Gregory Meeks, both Democrats, on what they called a "goodwill" mission.
He said that crucial to that process was transparency on the extent of past US snooping and clear rules of play in the future.
Murphy insisted that many in the United States also had deep concerns about overreach by the intelligence services and that there was "momentum" in Congress towards new curbs.
Murphy, who chairs the Senate Foreign Relations Subcommittee on European Affairs, later said in remarks to a public policy think-tank that President Barack Obama was taking Europe seriously.
"I know to many this looks like window-dressing," he said, referring to European scepticism about a comprehensive review of espionage practices ordered by Obama. "But this president is sincere."
Murphy said Europe and the United States could try to harness the crisis to create a "renaissance" in transatlantic ties with a sweeping free-trade pact and frank talk on the future of NATO.
And Murphy said he was aware that Germany was marked by its particular history, with gross abuses of state surveillance by the Nazis and the East German communists.
But he said Europe needed to understand that fears sparked by the attacks of September 11, 2001, shaped the US debate on security and spying.
Congressman Meeks, who with Murphy also met the German interior minister and members of parliament in Berlin, said he was aware of the "hurt" in European capitals.
"There are some very tough conversations that we have to have," he said."I think the president of the United States is serious about having them."
Germans have reacted angrily to revelations that emails, phone calls, web searches and other data may have been hoovered up by US intelligence agents, as part of widespread espionage that has also strained Washington's ties with other partners.
After meeting the US delegation Monday, Thomas Oppermann, the Social Democrats' parliamentary group leader and chairman of the secret service oversight committee, said the US espionage affair was "not over".
"We expect further light to be shed," he said, adding there had been agreement between the parties "that the completely out-of-hand practice of bugging by the NSA must finally have limits".
MP Michael Grosse-Brömer of Merkel's conservatives said he told the delegation "a clear and legally convincing agreement on intelligence cooperation and the protection of the private sphere of German citizens" was crucial.
Merkel called in parliament last week for answers over "grave" US spying accusations which, she said, were testing transatlantic ties, including fledgling US-EU trade talks.