UNITED NATIONS (AP) _ While Congress is demanding cuts in U.S. contributions to the United Nations, Sen. Jesse Helms is scrutinizing high rent subsidies being paid with tax dollars for the nation's U.N. diplomats.

The subsidies, totaling $1.8 million a year for 45 members of the U.S. mission staff in New York, include the $10,921 a month being paid for the deputy chief delegate's three-bedroom apartment.

The luxury apartment, on the 28th floor of a tower by the East River, is occupied by Herbert S. Okun, deputy to Vernon Walters, the U.S. ambassador to the United Nations.

In addition to his rent, the government pays $1,658 per month for a fulltime servant at Okun's apartment. He also is reimbursed for ''representati on expenses'' including food and drink. The September-Decenber bill was $6,281.

Okun contributes 5 percent of his salary, or $300 a month, to the costs which total about $14,000 a month.

These figures and others are contained in a memorandum sent by the undersecretary of state for management, Ronald Spiers, to the North Carolina Republican in answer to a set of Helms questions about expenses at the U.S. mission.

The memo said another Walters deputy, Patrica Byrne, lives in a three- bedroom apartment for $6,990 per month.

Walters lives in the same four-bedroom suite at the Waldorf Astoria Hotel occupied by his predecessors. But the hotel is upping the rent this year from $185,000 to $235,000.

Mission spokesman Richard C. Hottelet, in a letter defending the subsidies, said the rent for Okun ''is surely high. But half a dozen men or women of his rank from other countries live in the same building.''

He said the officials who receive housing payments come to New York ''because their government needs them here.''

''One cannot expect the highest quality performance at bargain-basement prices either in private business or in government,'' he said.

''The work of diplomacy is not done only in offices, public places and on the telephone, but also requires a network of personal contact and evaluation which can best be built in a social setting,'' he said.

The General Assembly is currently in session to consider measures to deal with a financial crisis caused by U.S. cuts of about $70 million in its contribution to the 1986 budget.

The cuts were mandated by Congress after criticism that the United Nations spends wastefully and that the major donors, including the United States which has been paying a quarter of the U.N. budget, do not have enough say over spending.