Hunger and homelessness rose sharply in urban America last year, despite improvements in the national economy, according to a report released yesterday by the U.S. Conference of Mayors.

The report, based on a survey of 25 major cities, said that the "new poor" created by the 1982 recession have not disappeared and that the number of families with children seeking food and shelter increased "significantly" last year.

The report blamed unemployment, a shortage of low-income housing, low welfare benefits, changes in mental health policies and tightened eligibility requirements for food stamps and public housing for the increases in hunger and homelessness.

Demand for emergency food rose an average of 28 percent in cities surveyed and that for shelter an average of 25 percent, the report said. Detroit was the only city surveyed that reported no increase in hunger; only three cities -- Minneapolis, Hartford and San Antonio -- reported no increase in homelessness.

The report said homeless people are "routinely" turned away from overcrowded shelters in half the cities surveyed, and 17 percent of the demand for emergency food "goes unmet."

The report was released as mayors of the nation's largest cities began arriving here to attend their annual winter meeting and lobby for continued federal spending on domestic problems.

The 25 cities were selected because the mayor of each serves on a task force on hunger and the homeless headed by Boston Mayor Raymond Flynn. They range in size from New York and Chicago to Yonkers, N.Y., and Charleston, S.C., and in economic health from Phoenix, with a 3.5 percent unemployment rate, to San Juan, with a 13.7 percent rate.

Unemployment in 11 of the cities decreased or was unchanged in 1985. But 22 of the cities said the official unemployment rate "does not accurately reflect the actual unemployment problem" because it fails to take into account the underemployed and those who have given up looking for jobs.

Chicago officials reported that "the new poor are not being recalled to their former jobs because their former plants are not being reopened." Seattle officials said the recovery has yet to reach their city.

"In none of the cities surveyed has the economic recovery lessened the problem of homelessness," the report said, quoting Boston officials as saying that "the recovery has exacerbated the situation as the increased attractiveness of the city has tighted the housing market."

Reflecting trends that began with the recession, 85 percent of the cities reported an increase in homeless children. Overall, the report said 60 percent of the homeless are single men, 27 percent are families with children and 12 percent are single women. About 33 percent of the homeless are considered to be chronically mentally ill.