The Washington Post newspaper published documents on Wednesday which suggest that the US National Security Agency (NSA) secretly broke into key main communication links from Yahoo and Google data centers around the world.

Citing the documents, leaked by former NSA contractor Edward Snowden, the newspaper reported that the agency was able to collect data from hundreds of millions of user accounts, including from Americans using a program dubbed MUSCULAR.

The program, operated jointly with the NSA's British counterpart GCHQ (UK Government Communications Headquarters), allows the agencies to intercept data flows from the fiber-optic cables used by the US Internet giants.

One top secret document dated January 9, 2013, revealed that around 181 million records were collected in the prior 30 days, ranging from metadata on emails to content such as text, audio and video.

The document indicates that the intercept takes place outside the United States, giving the NSA more latitude to collect data. Within the US, the NSA is obligated to acquire a court order, the Post said. As such, it differs from a separate data-gathering program, called PRISM, which uses court orders to compel Yahoo, Google and other Internet companies to provide data.

Google, Yahoo express concern

Asked about the allegations, NSA chief General Keith Alexander said he was unaware of the report, but said they appeared to be inaccurate.

"That [activity] to my knowledge, this never happened," he said at the conference sponsored by Bloomberg Television.

"We are not authorized to go into a US company's servers and take data. We'd have to go through a court process for doing that," Alexander added.

Google and Yahoo, meanwhile, have reacted with anger, denying any involvement in the surveillance activity.

"We have long been concerned about the possibility of this kind of snooping, which is why we have continued to extend encryption across more and more Google services and links, especially the links in the slide," Google chief legal officer David Drummond said in a statement.

"We do not provide any government, including the US government, with access to our systems. We are outraged at the lengths to which the government seems to have gone to intercept data from our private fiber networks, and it underscores the need for urgent reform," Drummond said.

In a statement to news agency AFP, Yahoo said: "we have strict controls in place to protect the security of our data centers, and we have not given access to our data centers to the NSA or to any other government agency."

Following the revelation, Germany's DJV journalists' union in a statement Thursday warned journalists against using Yahoo or Google for digitial communication or conducting research. The head of the organization, Michael Konken, said journalists should be aware of the consequences of using the services.

"There are definitely other search engines and email providers that we know from current knowledge are considered safer," he said.

Germany addresses spy reports

The latest revelations come amid growing international anger over the extent of NSA surveillance.

Earlier on Wednesday an official German delegation met intelligence representatives at the White House to discuss reports that the NSA had monitored Chancellor Angela Merkel's phone.

The talks were the first high-level face-to-face meetings between the allies since it was reported that the communications of Merkel and more than 30 other leaders were targeted by the agency.

In attendance were Merkel's National Security Advisor, Christoph Heugsen, and Secret Service Coordinator Günter Heiss. The US officials included White House National Security Advisor, Susan Rice, assistant to President Barack Obama for homeland security and counterterrorism, Lisa Monaco, James Clapper, director of national intelligence, and John Inglis, deputy director of the National Security Agency.

Rice tweeted afterwards that the dialogue was "constructive" and would continue in coming weeks. "Vitally important that we are staying focused on working together to protect our homelands and address global threats we face," she wrote.

Ahead of the meeting Merkel's spokesman Steffen Seibert told reporters that the goal of Wednesday's meeting in Washington was to clarify media reports of US spying, and to build "a new basis of trust and new regulation for our cooperation in this area."

Europe-US tension

A number of European countries have voiced their concern following the media reports that the NSA has been spying on their citizens and leaders.

In Spain, it was reported this week that more than 60 million telephone calls were picked up by the NSA in a month.

If proven to be true, Spanish Prime Minister Mariano Rajoy said it revealed a kind of behavior that was "inappropriate and unacceptable between partners and friends." Madrid's intelligence chief is due to address parliament over the allegations.

Meanwhile, Italy's Panorama magazine reported on Wednesday that the NSA had listened in on Vatican phone calls. The alleged eavesdropping occurred before cardinals elected the new pope in a March conclave.

The United States has sought to distance itself from claims the NSA collected data on communications across Europe. On Tuesday Keith Alexander told the House Intelligence Committee that the reports were "completely false."

According to UN spokesman Martin Nesirky, the US has also asserted it is not spying on internal United Nations communications, nor will it do so in the future. However as with the allegations relating to Angela Merkel, it did not clarify whether it had done so in the past.

ccp,dr/jr (AFP, Reuters, dpa)