The allegations were made in German news magazine Der Spiegel on Wednesday, based on information leaked by rogue US analyst Edward Snowden. It prompted a phone call from Merkel to US President Barack Obama, while Germany's Foreign Minister Guido Westerwelle then took the unusual step of calling US ambassador John B. Emerson to a meeting on Thursday afternoon. Westerwelle emerged to tell reporters “we need the truth now.”

Little more light was shed on the allegations when White House spokesman Jay Carney fronted a news briefing later in the day. He said Merkel's phone was not being tapped, but refused to say whether US intelligence had done so in the past.

"We are not going to comment publicly on every specified alleged intelligence activity," Carney said. "The president spoke with Chancellor Merkel, reassured her that the United States is not and will not monitor the chancellor's communications."

He added: “We acknowledge that the United States gathers intelligence much as other nations gather foreign intelligence.”

Merkel addressed the allegations before attending Thursday's European Union summit in Brussels, saying "trust needs to be re-established" in relations with Washington.

Allegations have 'caused tensions'

Carney acknowledged the allegations would test the relationship between the two nations: "The revelations that have appeared of late have obviously caused tensions in our relationships with some countries, and we are dealing with that set of issues through diplomatic channels," he said.

Wednesday's news has revived scrutiny of Merkel's handling of previous allegations of NSA spying on German citizens.

"The report that the chancellor's mobile phone was also tapped shows how absurd the attempt was to end the debate about the surveillance of everyday communications in this country," Germany's commissioner for data protection and freedom of information, Peter Schaar, told the regional daily Mittelbayrische Zeitung.

"In light of the new revelations it was downright irresponsible not to have pushed harder to get to the truth," behind the allegations, Schaar added.

Schaar was referring to a statement issued by the head of the chancellery, Ronald Pofalla, back in August, when he publicly declared the controversy over alleged NSA spying activities in Germany to be over. The chancellor herself later made a similar statement in an interview with ZDF public television.

ph/av (AP, AFP, Reuters, dpa)