The National Security Agency's (NSA) surveillance network has the capacity to reach around 75 per cent of all US internet communications in the hunt for foreign intelligence, according to the Wall Street Journal.

Citing current and former NSA officials, the newspaper says the 75 per cent coverage is more than officials have publicly disclosed.

The Journal says the agency keeps the content of some emails sent between US citizens and also filters domestic phone calls made over the internet.

The NSA's filtering, carried out with telecom companies, looks for communications that either originate or end abroad, or are entirely foreign but happen to be passing through the United States, the paper said.

But officials told the Journal the system's broad reach makes it more likely that purely domestic communications will be incidentally intercepted and collected in the hunt for foreign ones.

The Journal says these surveillance programs show the NSA can track almost anything that happens online, so long as it is covered by a broad court order.

Edward Snowden, a former NSA contractor, first disclosed details of secret US programs to monitor telephone and internet traffic earlier this summer.

The NSA could not be immediately reached for comment but says its surveillance is legal.

Reuters