The federal government is encountering substantial problems in trying to find new quarters for the 600 men and women who sleep nights at a squalid downtown shelter for the homeless that the government plans to close on Saturday, the official overseeing the relocation said yesterday.

Deputy Undersecretary of Health and Human Services Dixon Arnett, who each day this week has put off a planned announcement of alternative shelters, said, "Clearly there has been difficulty.

"It's not so much a problem in finding the physical sites," Arnett said. "It's been the difficulty in getting people who control the facilities to agree to have them used. It's a sticky issue at best, one in which there's not a lot of political reward."

Arnett said he now hopes to announce new shelter locations today, but conceded, "If we are able to put a list together by today , we will have to overcome a heck of a lot of difficulties."

Arnett said the government's "current plan" is still to close the decrepit 800-bed shelter at 425 Second St. NW on Saturday. "Having the Saturday deadline pushing everyone is a good thing," he said.

U.S. District Judge Charles R. Richey told the government last week it could carry out its plan to close the facility, but not before finding alternative shelters for the homeless who sleep there and devising a long-range plan "to eliminate homelessness in the nation's capital."

The Community for Creative Non-Violence, which operates the facility, has sued the government to try to force it to rehabilitate the World War II-vintage building. President Reagan, prompted by a 51-day fast last fall by CCNV leader Mitch Snyder, agreed two days before his reelection to turn the facility into a "model physical shelter." But negotiations over the extent of the renovations broke down in mutual recriminations.

Snyder, told of Arnett's admission that federal officials now are having difficulty finding new shelters, sarcastically remarked, "Isn't that honest of them?"

"Everything's falling apart," he said. "They're in trouble right now. We're going to stay in business until either Congress or the courts make the president do what he promised."

The CCNV has asked the U.S. Court of Appeals to block the federal government from closing the shelter on Saturday and a three-judge panel, in an unusual summertime session, is scheduled to hear the case today.

Meanwhile, the government, in opposing CCNV's request, said yesterday that it may appeal Richey's requirement that it find the alternative shelter space. Government attorneys, in a legal brief filed with the appellate court, said, "We question the basis for those portions of his order that appear to restrain action by the government to 'reclaim' the shelter," which is owned by the General Services Administration.

Arnett said government attorneys believe that Richey's conditions "are extralegal, which is to say out of his jurisdiction."

Nonetheless, the HHS official said the government is still trying to find alternative shelters. "We want to be able to do this as a matter of cooperation, not as a specific matter of law," he said.

The government has pledged that it will spend $2.7 million on the relocation effort, the amount it had planned to use to renovate the Second Street shelter. But Snyder has repeatedly said that amount is woefully lacking.

The Coalition for the Homeless, an advocacy group for the District's homeless and an organization at odds with Snyder's CCNV, has advanced several possible shelter sites to federal officials in recent days, but said that it would cost $10.8 million to buy and operate them for a year.

However, officials of several of the would-be shelter sites mentioned in the coalition's report said they either had not discussed their use with the coalition or had not done so for several months.

One site was identified as Brady Hall, a one-time dormitory at Eighth and Varnum streets NE that is owned by Catholic University.

But Vincent Lowe, the university's vice president for administration, said any suggestion that the building might be turned into a shelter for the homeless "was done without our consent or knowledge. It's for sale, but it's not at all suitable for students or homeless who don't have cars. It's four miles from downtown."

Elizabeth Huguenin, president of the coalition, said the report was only intended as "a draft proposal" listing buildings "that we were supposed to look into further.

"I'm confident that when the Second Street center closes on Saturday, there will be places to go," she said. But she, like Arnett, refused to identify any of the locations.

Meanwhile, Rep. Michael D. Barnes (D-Md.), chairman of the House District Subcommittee on Government Operations and Metropolitan Affairs, asked HHS Secretary Margaret M. Heckler yesterday to delay closing the Second Street facility past Saturday and requested that "the administration honor its commitment to provide a model shelter for the homeless in the District of Columbia."