As many as 2,000 children have stopped going to public schools in Alabama as the toughest immigration law in the country forces families into hiding for fear of deportation, rights advocates say.

"The impact on immigrant children and families has been nothing short of terrifying," said Ali Noorani, head of the National Immigration Forum, a rights organization based in Washington DC.

The US Justice Department on Friday pressed a federal appeals court in Atlanta to block implementation of the Alabama law, as civil rights activists like Noorani warned of a humanitarian crisis in the making.

The Justice Department acted after some of the law's most controversial provisions were upheld last week by a US district court judge, setting the stage for a confrontation that echoes battles in southeastern states such as Arizona and Georgia.

Alabama's law, HB 56, regarded as the toughest yet, makes it a crime to be in the country without papers, requires schools to check the immigration status of students, and empowers police to ask people for their documents in routine stops.

In arguing against it, US federal attorneys have warned it would expose people in the country legally to discrimination and lead parents to keep children away from school to avoid deportation.

Nevertheless, said Noorani, the law's provisions "are moving towards implementation."

"The crisis in Alabama is unfolding and our nation's children are caught in the cross fire. We shouldn't be playing politics with children's lives," he said in a teleconference Friday with other opponents of the law.

"We've heard that there are a couple of thousands that have stayed home from school," said Mary Bauer, legal director of the Southern Poverty Law Center.

"There are many, many families that have mixed status, so it is impossible to know the ties that people have. So US citizen's kids are affected because a family member is undocumented."

She said that while immigrants account for only about two percent of Alabama's population that still amounts to "thousands and thousands of children and families."

State education officials were unable to provide state-wide figures for unexplained absences of Hispanic children in public schools.

But they are concerned enough that the interim superintendent of the state's education department, Larry Craven, assured parents on Tuesday that information would be used only for statistical purposes and not to identify individual students.

"Alabama’s public schools welcome all children regardless of their race or ethnic background, birthplace or birthplace of parents and the Alabama Department of Education (ALSDE) is committed to all students and parents to provide a safe and civil environment in which students can pursue their education," he wrote.

But immigrants and social workers who advocate for them remain wary of the state's intentions.

Father Jack Kane, a Catholic priest from Opelika, Alabama, noted that the immigration law was toughened as it moved from the state senate to the lower house.

Churches, for instance, were supposed to have been exempted from prosecution for providing services like driving illegal immigrants to the hospital. But the lower house removed those protections.

"That's one of the things that Judge Blackburn did enjoin, thank goodness for that," Kane said, referring to US District Judge Sharon Blackburn. "But it goes to the intentionality of the people who are making the laws," he added.

Nationwide, there are an estimated 11 million undocumented immigrants in the United States, most of them Hispanics.

The Pew Hispanic Center, the top US institute for the study of the Hispanic community, estimates that in Alabama there are 130,000 undocumented immigrants in Alabama.

Although Arizona proposed the state's first immigration law that criminalized undocumented aliens, its most controversial provisions, like empowering the police to question a person's immigration status, were blocked by the courts.

But other states have followed suit with their own laws in defiance of Washington.

The legislation has not been without cost. In Georgia and Alabama a shortage of workers has appeared in key sectors including agriculture as immigrants head for the borders.