The White House has denied it is listening in to Merkel's private conversations.
The comments come as a row over alleged spying by the NSA deepens. The issue dominated a summit of the European Union's (EU) 28 leaders in Brussels late on Thursday and is set to remain a key topic as they meet again on Friday. Members of the European Parliament have been in uproar in the past few months over reports in German newspaper Der Spiegel that the NSA bugged EU offices.
(Read More: White House: US not monitoring Merkel's calls)
Schulz said that the U.S. ambassador for Germany had "no explanation" for the revelations, giving a clear sign, according to Schulz, that U.S. government and officials have no control over the secret services.
"I think this is a question of respect," he said. "The best way is to bring light into the darkness."
(Read More: US fails the European allies' trust test)
The lawmakers have lauched a series of inquiries into the spying allegations. On Wednesday, they voted in favor of suspending a controversial financial data sharing agreement between the EU and the U.S. aimed at tracking terrorists' funds called the Terrorist Finance Tracking Program (TFTP).
The revelations are part of the vast leaks made by former U.S. data analyst Edward Snowden. The U.K.'s Guardian newspaper reported on Thursday the NSA monitored the phone conversations of 35 world leaders after being given the numbers by an official in another U.S. government department.
(Read more: This is like the 'Cold War': Europe fumes over US spying)
On Wednesday, Merkel called President Barack Obama to demand clarification about whether her phone had been tapped, warning that such an action would amount to a "serious breach of trust", according to government spokesperson Steffen Seibert. The White House responded by insisting in a statement that it "is not monitoring and will not monitor" Merkel's phone.
Germany and France announced on Thursday that they are now seeking a "mutual understanding" with the United States on cooperation between their intelligence agencies. Other EU member states join that agreement. Reuters also reported that Germany's foreign minister warned that the country's historic friendship with the U.S. is at risk unless Washington lays "all cards on the table".
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