By Alina Selyukh

WASHINGTON (Reuters) - At least a dozen U.S. National Security Agency employees have abused secret surveillance programs in the past decade, most often to spy on their significant others, according to the latest findings of the agency's internal watchdog.

In a letter to the Senate Judiciary Committee's top Republican, Charles Grassley, NSA Inspector General George Ellard outlined 12 instances of "intentional misuse" of the agency's intelligence gathering programs since January 1, 2003.

Grassley had asked the NSA internal watchdog to report on "intentional and willful" abuse of the NSA surveillance authority as public concerns mount over the vast scope of the U.S. government's spying program.

The agency's 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."

The NSA inspector general, in the letter dated September 11, detailed 12 investigations that found the NSA's civilian and military employees used the agency's spying tools to search for email addresses or try to snoop on phone calls of current or former lovers, spouses and relatives, both foreign and American.

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In one instance, a military member queried six email addresses of a former girlfriend, an American, on the first day of having access to the data collection system in 2005.

In another instance, a U.S. government-employed foreign woman suspected an NSA civilian employee, who was her lover, of listening to her phone calls. 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.

In at least six of the 12 instances reported by the inspector general, the matters were referred to the Department of Justice. In several instances, the violators resigned or retired from their jobs before being disciplined.

(Reporting by Alina Selyukh; editing by Christopher Wilson)