A Pentagon official has informed Uruguay's military government that its request to buy American munitions will not be approved "until existing human rights issues with your country have been resolved."

The strongly worded letter, signed by Maj. Gen. Richard E. Cavazos was made public in response to a request under the Freedom of Information Act by the Washington Office on Latin America. The office is a church-supported group that takes a special interest in human rights issues.

The letter, dated June 30 of last year, refers to Uruguay's "request for munitions for the Uruguayan army for 1978." In it, Cavazos urges the Uruguayan government to take measures "well beyond what Uruguay's critics are asking, in order to reaffirm to the world the durability of Uruguayan democracy."

He added: "I believe the chances for approval of your munitions request is entirely dependent upon the initiatives which your government decides to take in improving human rights."

The Uruguayan government has been a major target of congressmen and groups pressing the Carter administration to strictly enforce its policy of cutting off aid to governments that violate human rights.

Congress barred military aid to Uruguay in 1976 because of reports of torture and other abuses there. The following year the administration submitted no request for military aid to Montevideo.

The military aid cutoff did not affect commercial sales of American arms and material to the Uruguayan military.

Cavazos letter, addressed to Uruguayan military attache Brig. Gen. Luis Queirolo does not list the specific type of munitions the Uruguayan army had requested. It does refer however, to U.S. policies against the sale of white phosphorus and law enforcement equipment such as .38-calibre special pistol ammunition.

It says sales of white phosphorus, a napalm-like substance used against tanks, are prohibited to any country, and sales of police equipment will not be approved."

A Uruguayan source aid the army had requested approval of commercial sales of items like radio equipment, ammunition, roadbuilding equipment, uniforms and field equipment like canteens and compasses. He denied that Uruguay had asked to buy white phosphorus. The source expressed "alarm that private correspondence is violated in this way."

Uruguay, a country on the east coast of South America with a long tradition of elected civilian governments, came under military rule in 1973 after the armed foces began an offensive against the leftist Tupamaro guerrillas.

In the letter, Cavazos also cautioned the Uruguayans that support for the administration's human rights policy is much broader than they thought.

The support, he said, "is not limited to a few liberal members of Congress." Instead, "the U.S. public, the great majority of Congress, and the administration . . . are firmly committer-American region of the Pentagons' around the world."

Cavazos, who was director of the inter-American region of the entagons' International Security Affairs section when he wrote the letter, is now commander of the 9th Infantry Division at Ft. Lewis, Wash.