Despite tough immigration changes, a weak economy and a slack job market, immigrants continue to flock the US by the millions, breaking historic patterns in which immigration levels rise and fall with the economy, says the Washington-based Centre for Immigration Studies (CIS), a group that advocates tighter immigration enforcement.

Immigrants now account for 13 per cent of the country's population, the CIS report said quoting as-yet-unpublished US Census Bureau data.

The 4.3 million increase in the number of immigrants since 2000 included over two million illegal aliens, who now total about 10 million or 30 per cent of the immigrant population, CIS said.

"The idea that immigration is a self-regulating process that rises and falls in close step with the economy is simply wrong," said Steven Camarota, CIS Director of Research and the author of the report released Tuesday.

"Today, the primary sending countries are so much poorer than the United States, even being unemployed in America is still sometimes better than staying in one's home country."

Camarota said the countries primarily represented among the nation's immigrant population are much poorer than the primary sending countries in the past.

The United States' much higher standard of living, Camarota said, exists even during recessions, noting that people come to the US to join family, avoid social or legal obligations, take advantage of social services, and enjoy greater personal and political freedom.

The study found that immigrant flows didn't slow even after the unemployment rate among immigrant adults rose from 4.4 per cent in 2000 to 6.1 per cent in 2004. About half of those newcomers were illegal immigrants, the report estimated.

"It supports the view that people now largely come to America not for economic reasons but for bigger issues," Camarota said. "They want to be with family members, they like our public services or they are avoiding some obligation such as serving in the military back home."

"Even a prolonged economic downturn is unlikely to have a large impact on immigration levels. If we want lower immigration levels it would require enforcement of immigration laws and changes to the legal immigration system," he said.

States with the largest increase in their immigrant population were Texas, Georgia, North Carolina, New Jersey, Maryland, Washington, Arizona, and Pennsylvania.

The report comes as the Bush Administration revives a proposal to offer temporary "guest worker" visas to illegal immigrants working in the US, a proposal Camarota opposes.

"Even if six per cent of immigrants are unemployed, that means 94 per cent of them are working," he said. "The vast majority of people coming here have jobs."