A senior security officer at the United Nations suffered a deadly gunshot wound to his head in an apparent suicide in an employees' lounge above the General Assembly, sources said.

The death of Michael Halton, 41, stunned friends and colleagues as well as bystanders gathered in the building for a World AIDS Day function with Jermaine Jackson, boxer Leila Ali and songstress Ashanti.

Halton, a popular 16-year veteran of the U.N. security force, was apparently depressed over marital problems and had recently begun taking an anti-depressant drug, police sources said.

He was the first person to die violently at the U.N. since it was founded more than a half-century ago.

Authorities say Halton showed up for work yesterday at 8:45 a.m. and manned his usual post at the entrance to the building near 42nd Street, where the public gathers for tours through the building.

At about 10 a.m., Halton took a brief break and headed for the third-floor lounge. When he failed to return to his post an hour later, co-workers went looking for him and made the grim discovery.

He was found sprawled near a chair with his 9 mm Glock next to his body, sources said.

Halton's boss, Michael McCann, fought back tears as he described Halton as an affable colleague who "always had a smile."

"He was simply a great guy," a co-worker said, adding that Halton was married with two kids.

Sources said despite Halton's upbeat demeanor at work, he had recently been depressed and, unbeknownst to his bosses, had been taking medication during the past week.

Stunned and sobbing neighbors and relatives gathered at Halton's tidy two-story home in West Sayville, L.I., which was festooned for the holiday season with lights, a wreath on the front door and two statues from the "Nutcracker Suite" on the lawn.

"Mike was always playing ball with his kids out front or walking with his wife," said a neighbor who did not want to be identified.

Another neighbor, who used to baby-sit the Halton children, said: "I never would have expected this. It's a shock."

Two other U.N. security guards died this year - one in the Staten Island Ferry tragedy last month, the other from cancer.