National Security Agency agents at the Threat Operations Centre
Staff at the US National Security Agency's Threat Operations Centre have used technology to spy on loved ones, the NSA said.

US intelligence agents tasked with monitoring terrorist and criminal communications have turned their sights on targets closer to home.

Officers at the US National Security Agency have used the sophisticated eavesdropping technology at their disposal to spy on loved ones, agency officials have confirmed.

At least ten cases have emerged of NSA staff spying on a love interest, with the practice even gaining its own ironic codename within the agency: "LOVEINT".

Instances of the abuse have numbered an average of one case a year for ten years .

Democratic Senator Dianne Feinstein said the NSA had informed Congress of "isolated cases" of LOVEINT.

The incidents involved foreigners rather than US citizens, said Feinstein, who chairs the Senate intelligence committee.

"Clearly, any case of noncompliance is unacceptable, but these small numbers of cases do not change my view that NSA takes significant care to prevent any abuses and that there is a substantial oversight system in place," said Feinstein.

"When errors are identified, they are reported and corrected."

Feinstein said she had seen no evidence of violations involving the use of the NSA's domestic surveillance infrastructure.

The LOVEINT violations involved overseas communications such as spying on a partner or spouse, said officials.

Cases of "LOVEINT" constitute the majority of the reported instances of willful misconduct by NSA employees, they added.

In each instance, the employee was punished either with administrative action or termination.

The agency is thought to be responsible for at least 3,000 violations of privacy rules a year.

Those behind the incidents are on occasion believed to have confessed to the abuses themselves during polygraph tests to renew their security clearance.

NSA Chief Compliance Officer John DeLong said he did not have the exact figure for cases of LOVEINT, but said cases were "very rare", and none violated key surveillance laws.

"NSA has zero tolerance for wilful violations of the agency's authorities," the agency said, adding that it responds "as appropriate".

The revelations come amid an international outcry following revelations of a vast government surveillance programme, after leaks by the former CIA computer expert Edward Snowden were published by newspapers including the Guardian and New York Times.

Snowden, who is wanted in the US on espionage charges, has been granted temporary asylum in Russia.

The partner of Guardian reporter Glenn Greenwald was detained for nine hours at Heathrow last week as he attempted to carry information to Greenwald in Brazil. David Miranda's laptop and mobile phone were confiscated, and criminal proceedings later launched.

Further revelations over the extent of government surveillance are expected.