While boasting some of the world's fastest economic development, the countries of Asia face major environmental problems that threaten their future resources, according to a new United Nations report.

The report, issued here recently, singled out as the most serious threat the rapid shrinkage of the continent's economically and ecologically vital tropical forests, which it said are "disappearing at an alarming rate." At present rates, the study said, Malaysia could lose all of its forests in 12 years, the Philippines in 14 years and Thailand in 21 years.

Asia's rain forests are regarded as the most productive in the world and are a major basis for economic development in many countries. The report said the reduction in forest areas already had caused such problems as increased seasonal flooding with loss of life and property, water shortages during the dry season, increased erosion of agricultural land and mounting silt deposits in rivers and coastal waters.

Other major environmental problems cited in the study included declining productivity of farmlands, deterioration of water quality in seas and waterways and increasingly serious air pollution in some of the region's largest cities.

These issues are among the topics being discussed at the annual session of the U.N. Economic and Social Commission for Asia and the Pacific, which winds up this weekend. The Bangkok-based commission includes 44 countries bounded by Iran, Mongolia, Japan and the Cook Islands, an area including more than half the world's population.

According to Shah Kibria of Bangladesh, the commission's executive secretary, many of the continent's environmental problems can be traced to poverty. At a recent seminar on environment and development, he noted that of the 2.4 billion population in the commission's region in 1980, about 969 million were classified as below the poverty line. They comprised 84 percent of the world's total poor.

The region's extreme poverty compels many of the poor to overexploit forests, farmlands and waterways to survive, Kibria said. He called the situation a "vicious circle in which it is difficult to determine whether the actions of the poor lead to environmental damage or whether lower environmental carrying capacity leads to poverty."

As one result, tropical forests are disappearing at a rate of 2 percent a year throughout the region, according to Kibria. He added, "One estimate is that the region will lose 70 percent of its forest cover by the year 2000 unless appropriate long-term measures are taken now."

The U.N. report, the first of its kind, said 378 million people in the area were threatened by man-made and natural processes that turn productive land into deserts. It said large areas of China, India and Iran and parts of Afghanistan and Pakistan were affected by the process.

Another major problem, according to the study, is a continuing decrease in marine resources such as edible fish at a time when there is a growing need for them. Because of projected population increases and reduced land productivity, it said, the region's annual fish catch of 30 million tons must increase to 54 million tons by the turn of the century to meet protein requirements.

The study also reported these environmental health findings:

* Air pollution from automobiles in Bangkok gives the city a carbon monoxide concentration of between 13.9 and 32.4 parts per million, compared to the World Health Organization's standard of 9 ppm. (By contrast, carbon monoxide levels in Washington range between 5 and 20 ppm.)

* Air and water pollution in Bombay pose serious health hazards, with an average of 190 tons of sulfur dioxide emitted into the air daily and more than 300 million gallons a day of untreated industrial waste water discharged along the coastline.

* Of the 8 million population of Manila and its environs, 75 percent lack potable water and 90 percent of the area's homes are not connected to sewers.

* High concentrations of the toxic pesticide DDT were found in water and milk in southern Iran near the Abadan oil refining center, and a U.N. advisory mission was told that bread there was baked with a petroleum derivative that included about 150 chemical compounds, several of which were carcinogenic.