Confirmation that the U.S. National Security Agency monitored the cellphone of German Chancellor Angela Merkel would damage Washington's credibility, the co-chair of the parliamentary panel that oversees Germany's intelligence services said Thursday.

Thomas Oppermann, a senior member of the main opposition Social Democrats, commented before a special meeting of the intelligence committee convened after the government said it had information indicating Merkel's private communications were intercepted.

"If Chancellor Angela Merkel's cellphone has been monitored," he said, that would constitute both "clear harm to German interests" and a significant setback for U.S. "credibility."

Officials here have seen evidence tending to confirm their fears that NSA had access to the telephone conversations of Merkel as well as those of millions of ordinary German citizens, Oppermann said.

"We have apparently been deceived by the U.S. side," the lawmaker said, alluding to initial assurances from Washington following former NSA contractor Edward Snowden's revelations about the massive global surveillance.

Oppermann, who is seen as a possible interior minister if his party agrees to join a governing coalition with Merkel's Christian Democrats, said the spying issue threatens trade talks between the United States and the European Union.

"I cannot imagine any free-trade agreement ... unless these allegations are cleared up and a clear treaty basis is established in which the Americans promise to respect German sovereignty and to abide by the fundamental rights of the citizens," the lawmaker said.