The federal government has for the first time threatened to cut off federal funds for hospitals because the cities they have selected for new construction may diminish medical services available to minority groups.

The action, taken in separate letters from the Department of Health, Education and Welfare's Office for Civil Rights to two Gary, Ind., hospitals, was described by federal officials as part of a broader effort to expand enforcement of federal civil rights laws into new areas.

Part of the effort includes an attempt by the Office for Civil Rights to get other agencies in HEW to write civil rights considerations into their regulations.

If that is done, virtually every hospital in the country could have its construction plans reviewed to make sure that medical services for minority groups are not diminished or segregated.

At the moment, the Office for Civil Rights, in order to secure compliance in this area, must either negotiate an agreement with the hospital in question or institute administrative proceedings to cut off funds. Legal authority for such is not clear.

The Gary action, involving Methodist Hospital and St. Mary Mercy Medical Center there, was taken under the conditions of formal "assurances" or agreements negotiated earlie with each of the hospitals.

The assurances stated that each hospital would demonstrated to local and federal agencies - including HEW's Office for Civil Rights - that construction undertaken away from their central inner-city facilities would not adversely affect medical care in Gary, a predominantly black area.

Both hospitals - Mercy in 1973 and Methodist in 1976 - have opened satellite facilities in suburban communities about 15 miles outside of Gary. After the opening of those facilities - a 165-bed satellite addition to the 389-bed main Methodist hospital and a 184-bed satellite addition to the 285-bed main Mercy hospital - HEW secured agreements with the hospitals that they would get clearance on civil rights grounds before any additional construction was done.

Mercy hospital in September began construction of an $3 million extension of its satellite facility, involving an expansion of existing facilities and construction of a medical office building. Methodist is considering a $7.5 million expansion including a laundry and an outpatient clinic at its satellite hospital.

Tuesday, David Tatel, director of HEW's Office for Civil Rights, wrote to both hospitals asking that "no further action be taken until the construction plans are properly reviewed and approved by this department as required by the assurances."

In addition to the assurances in Gary, HEW last month signed a similar, but more detailed agreement with the Wilmington Medical Center, which is moving from inner-city Wilmington, Del., to a suburb eight miles hospital promised to provide transportation for innercity residents to the new facility.

The new enforcement effort by the Office for Civil Rights comes at the same time that proposed federal planning regulations have brought protests from communities across the country that they will force massive elimination of both, and, in some instances, closures of entire hospitals.

Tatel said in an interview yesterday that his office would be giving more attention to site selection. "To be honest, the Office for Civil Rights has not health enforcement activity at all. That's why we're the defendant in so many lawsuits. Now we're beginning to do it. The site selection provisions are there, and we're going to enforce them," he said.

Tatel said his office this year will be reviewing compliance with federal civil rights laws by federal fund recipients across the country, including hospitals. Additionally, he said, where complaints about site selection are received, "We'll apply the same standards we did in Wilmington and we are now in Gary."

Because of lack of previous enforcement, Tatel said, he could not estimate how big the problem is, "but we'll know soon."

Regulations written under the 1964 Civil Rights Act empower HEW to negotiate assurances with hospitals of construction plans appear to diminish services to minority groups.

According to Michael Middleton, director of the Civil Rights policy Office, the agency is attempting to have federal planning regulations now being written include specific authorization for the Office for Civil Rights to intervene where minority rights are not considered. Middleton said a similar effort is being made ith the Health Care Financing Administration, which administers both Medicare and Medicaid funds.