The US's National Security Agency has tapped directly into communications links used by Google and Yahoo to move huge amounts of email and other user information.

The revelations were made in a Washington Post report, based on secret NSA documents leaked by former contractor Edward Snowden.

The documents appear to show the agency has used weak restrictions on its overseas activities to exploit major US companies' data to a far greater extent than realised.

Previously reported programmes included those that allowed easy searches of Google, Yahoo and other internet giants' material based on court orders.

But since the interception in the newly disclosed effort, code-named MUSCULAR, occurs outside the US, there is no oversight by the secret intelligence court.

The newspaper said the operation gained access to a cable or switch that relayed the traffic through an unnamed telecommunications provider.

"We are outraged at the lengths to which the government seems to have gone to intercept data from our private fibre networks, and it underscores the need for urgent reform," said Google chief legal officer David Drummond.

Google said it had not been aware of the programme, although it recently began speeding its efforts to encrypt internal traffic.

Like other companies, Google and Yahoo constantly send data over leased and shared or exclusive international fibre-optic communication lines as they synchronise information.

The newly disclosed programme, operated jointly with Britain’s GCHQ, amassed 181 million records in one recent 30-day span, according to one document reported by the Washington Post.

'Valid foreign intelligence targets only'

An NSA spokesperson said in a statement the suggestion that the agency relies on a presidential order on foreign intelligence gathering to skirt domestic restrictions imposed by the Foreign Intelligence Surveillance Act and other laws "is not true".

"The assertion that we collect vast quantities of US persons' data from this type of collection is also not true," the spokesperson said.

"NSA is a foreign intelligence agency. And we're focused on discovering and developing intelligence about valid foreign intelligence targets only."

Asked at an event in Washington about the latest report, NSA Director General Keith Alexander said that he had not read it, but the agency did not have unfettered access to the US companies' servers.

"I can tell you factually we do not have access to Google servers, Yahoo servers," Mr Alexander said. "We go through a court order."