NEW YORK (AP) _ An Eastern Airlines jet was forced to take evasive action when it came close to a military aircraft over Long Island Sound, the Federal Aviation Administration reported today.

No injuries were reported in the incident, which was blamed on a mistake by an air traffic controller.

Mary Jo Byberg of the FAA office in Burlington, Mass., said the incident occurred at about 7:18 p.m. Tuesday and involved Eastern Flight 380, a Boeing 727 headed northeast from Washington to Boston, and a military C-141 on its way southwest to McGuire Air Force base in New Jersey.

She said that in his near-collision report, the Eastern pilot said that at 23,000 feet, ''he had to take evasive action because a military C-141 was within 300 feet vertically and a quarter of a mile horizontally.''

The FAA's own preliminary investigation, she said, concluded that ''the aircraft were 500 feet apart vertically, and three-quarters of a mile horizontally. The standard separation at that altitude, 23,000 feet, is 1,000 feet vertically and five miles horizontally.''

The Eastern pilot made a left turn.

The incident occurred over Long Island Sound, about 15 miles southwest of Bridgeport, Conn.

Byberg said, ''Our preliminary finding is that an operational error occurred, a mistake by an air traffic controller at the Boston air route traffic control center in Nashua, N.H.''

Robin Matell, a spokesman for Eastern in Miami, said airline officials had no report of the near-collision.