The National Security Agency has obtained access to hundreds of millions of Yahoo and Google accounts, sending millions of records a day from the networks to NSA headquarters, The Washington Post reports.

According to documents obtained from former NSA contractor Edward Snowden and interviews with knowledgeable officials, the NSA has secretly broken into the main communications links that connect Yahoo and Google data centers around the world and can collect anything from user accounts.

Last December, field collectors processed and sent back 181,280,466 new records that ranged from "metadata," which would indicate who sent or received e-mails and when, to content such as text, audio and video, according to The Washington Post.

The NSA exploits the data links through a project called MUSCULAR, which is operated jointly with the agency's British counterpart, GCHQ.

Under a separate program known as PRISM, the NSA also has front-door access to Google and Yahoo user accounts through a court-approved process.

White House officials and the Office of the Director of National Intelligence, which oversees the NSA, declined to confirm, deny or explain why the agency infiltrates Google and Yahoo networks overseas.

In a statement, Google said it was "troubled by allegations of the government intercepting traffic between our data centers, and we are not aware of this activity.

"We have long been concerned about the possibility of this kind of snooping, which is why we continue to extend encryption across more and more Google services and links," the company said.

At Yahoo, a spokeswoman 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."

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