International organizations say Thailand has become a regional magnet for foreign workers because of its prosperity and open economy. But they warn that the government faces serious challenges in dealing with large numbers of migrant workers and refugees.

The United Nations and the International Organization for Migration, IOM, Tuesday released what they say is the first-ever overview of international migration in Thailand.

IOM Representative Irena Vojackova-Sollorano, said Thailand has the most dynamic but complex situation in the region.

"All Southeast Asian countries mostly have a migration problem," she said. "Thailand has many migration problems, because it is a sending, it is a receiving, it is a transit country, everything at the same time, and in a magnitude which is just huge."

The report reveals that of the nearly two million estimated foreigners residing in Thailand, 1.2 million are registered workers from three neighboring countries, Cambodia, Laos and Burma. It says there are also more than 100,000 professionals working legally in the kingdom, nearly one-fifth of them from Japan.

There are no exact figures on illegal migrants, but the report quotes government figures, saying that a half million foreigners, from almost every country in the world, have over-stayed their visas.

The report adds that 150,000 Thai citizens work abroad, mostly in construction, and they remit $1.5 billion a year to their families in Thailand.

The report praises the Thai government for a program last year that allowed more than a million unregistered workers to apply for work permits. However, it notes that 90,000 refugees from Burma were excluded from the program.

They have been restricted to their camps and are forbidden from leaving to look for work.

The report also notes that thousands of ethnic Hmong refugees from Laos have gathered in northeast Thailand, reportedly drawn by a United States resettlement program last year. The Thai government wants to return them to Laos, but the Lao government says their identity must be verified first. IOM officials praised a joint identification program undertaken by the two governments.

One of the authors of the migration report, Jerrold Huguet, says the survey shows there is a large number of children of migrants in Thailand, nearly 100,000 of them, but that only 13,000 of those are registered in Thai schools.

"Because the legal employment age in Thailand is 15, we have a situation of more than a 100,000 children, most of whom are not in school and not legally permitted to work," he said.

Mr. Huguet concluded by saying that in the future Thailand's expanding economy will need more labor, but the population of young Thai workers is no longer growing because of lower fertility rates.

"If the Thai economy continues to expand and has a demand for young workers, necessarily that increase in young workers has to come from migrant workers, not from the Thai population," he added.

As a result, he says, there will eventually be considerable and permanent settlement of migrant workers in Thailand. And he urges the government to prepare for that prospect.