Poverty in America's cities has grown more persistent and concentrated over the last two decades and its victims have less chance of escape, according to a report by the National League of Cities.

The study describes a changing face of poverty since 1970, with the nation's poor more likely now to live in metropolitan areas and concentrate in low-income neighborhoods.It said the proportion of children living in poverty has increased and the likelihood of people climbing above the poverty level has declined.

"Poverty in cities is changing in quality and intensity," the report said. "It is more persistent among particular households, more concentrated within particular neighborhoods, and more isolated from traditional avenues of escape."

The league released its report, "Poverty in Cities," as nearly 3,000 municipal leaders gathered in Washington for the organization's annual winter meeting, which runs through Tuesday.

"Both the figures and the trends are alarming," Alan Beals, executive director of the National League of Cities, said in releasing the report. "Such conditions are devastating for those caught up in it, especially children."

The report, an analysis and compilation of previous research, said poverty in America declined through much of the 1970s, to a rate of under 12 percent at the end of the decade. But a resurgence brought the rate to more than 15 percent by 1985, and the average rate for the years 1980 through 1987 was over 14 percent, the league said.

The portion of the nation's poor living in metropolitan areas grew from 62 percent in 1979 to 70 percent in 1985 - an increase of 7.6 million people, with the majority living in central cities, it said.

The report said the persistence of poverty - that is, the length of time a person remains poor - has grown. It cited previous studies of the proportion of urban poor in a given year who escaped poverty the following year and said in the late 1960s, that figure was 32 out of 100.

Called the "escape probability," the figure rose to 37 out of 100 in the mid-1970s, meaning that poverty became less persistent, but began declining and was 23 out of 100 by the early 1980s.

"There has been a substantial decrease in the chances that a poor person will escape poverty within the following year," it said.

An increase was also recorded in the proportion of poor residents of big cities who live in neighborhoods that are classified as extremely poor - where at least 40 percent of residents live below the poverty line. Among the 50 largest cities, the concentration-of-poverty rate increased from 16 percent in 1970 to 24 percent in 1980.