The United States yesterday rejected North Korea's claim to a "military sea boundary" of 50 miles off its coasts for control of sea and air passage as a unilateral breach of international law.

North Korea also was cautioned that it cannot "diminish the obligations" it has under the 1953 Korean War armistice. "There will be no change in our operations in international waters and air space," said a spokesman for the U.S. administered United Nations Command in South Korea.

On Monday, North Korea asserted control over ships and aircraft in the zone extending into the Sea of Japan and the Yellow Sea. The claimed "military boundary" appears to extend over five island groups off the North Korean coast that the 1953 armistice assigned to South Korea's control, a step North Korea has disputed.

To undersocre South Korea's control, a step North Korea has disputed, ship with 465 passengers sailed with a naval escort yesterday to the South Korean-held island of Paengyong, about 10 miles off the North Korean coast. Officials said the trip was without incident.

Two South Korean navy ships normally escort such passenger vessels, United Press International reported, but the agency quoted military sources as saying seven naval escorts participated yesterday.

Carter administration officials said they doubt that North Korea seeks to provoke an incident at a time when the United States is working out plans to withdraw the bulk of its 33,000 ground troops from South Korea over a four-to five-year period.

"We are not trying to be provocative" either, said one administration source. "We are just continuing our operations; we are not backing off."

State Department spokesman John H. Trattner said North Korea's "unilateral" action "in no way curtails or diminishes" U.S. or United Nations Command rights in South Korea, or North Korea's own armistice obligations.

Trattner said that under international law, "we and many other nations exercise a freedom of navigation and overflight beyond the 12-mile territorial sea and we do not recognize the right of any nations unilaterally" to change it.

North Korea last year told a Japanese delegation it would challenge any vessel within 50 miles of its coast.

On Monday it put into effect a 200-mile economic zone off its coasts, asserting broader rights that the United States or other nations have announced for such zones.

The unusual, additional "military boundary", North Korea proclaimed "is up to 50 miles from the starting line of the territorial waters in the east sea [Sea of Japan] and to the boundary line of the economic sea zone in the west sea [Yellow Sea] between China and North Korea."

In this area, North Korea said, "acts of foreigners, foreign military vessels and foreign military planes are prohibited and civilian ships and civilian planes [excluding fishing boats] are allowed to navigate or fly only with appropriate prior agreement or approval."