The practice is known in intelligence world shorthand as "LOVEINT" and was disclosed by the NSA Office of the Inspector General in response to a request by the Senate Judiciary Committee's top Republican Charles Grassley for a report on abuses of the NSA's surveillance authority.

In one instance in 2005, a military member of the NSA queried six email addresses of a former American girlfriend - on the first day he obtained access to the data collection system. He later testified that "he wanted to practice on the system" and gained no information as a result of his queries.

In another instance, a foreign woman who was employed by the U.S. government suspected that her lover, an NSA civilian employee, was listening to her phone calls. She shared her suspicion with another government employee, who reported it.

An investigation found the man abused NSA databases from 1998 to 2003 to snoop on nine phone numbers of foreign women and twice collected communications of an American, according to the inspector general's report.

The NSA's spying operations have come under intense scrutiny since disclosures this spring by former NSA contractor Edward Snowden that the U.S. government collects far more Internet and telephone data than previously publicly known.

Many members of Congress and administration officials staunchly defend the NSA surveillance programs as a critical defense tool against terrorist attacks, but privacy advocates say the spying agency's authority has grown to be too sweeping.

Jameel Jaffer, deputy legal director at the American Civil Liberties Union, said the reported incidents of NSA employees' violations of the law are likely "the tip of the iceberg" of lax data safeguards, but that the laws guiding the NSA's spying authority in the first place are a bigger issue.

"If you only focus on instances in which the NSA violated those laws, you're missing the forest for the trees," he said. "The bigger concern is not with willful violations of the law but rather with what the law itself allows."

Most of the abuses detailed in the NSA inspector general's September 11 letter to Grassley were discovered through the agency's own audits, or self-reports and polygraph interviews with the employees. Their names were not disclosed.

According to the report, a female civilian NSA employee snooped on her husband's phone conversations after looking up a foreign number she found on his phone because she suspected him of cheating.

Yet another civilian employee, caught abusing the system by looking up the phone numbers of various foreigners she met socially, said she wanted to ensure she was not talking to "shady characters."

In at least six of the 12 instances reported by the inspector general since January 1, 2003, the matters were referred to the Department of Justice.

In several instances, the violators resigned or retired from their jobs before being disciplined. Others were demoted, given extra days of duty, had their pay cut, and had their access to databases revoked, the report said.

(Reporting by Alina Selyukh; Editing by Christopher Wilson and Marguerita Choy)