MONTGOMERY, Alabama - The US Justice Department is challenging a new Alabama immigration law that would let police detain people stopped for traffic offenses if they are suspected of being in the country illegally.

The Alabama law is described as one of the toughest of its kind nationwide.

Last year, the Justice Department obtained a preliminary injunction against a similarly stringent Arizona immigration law.

The Justice Department filed a complaint Monday in federal court in Birmingham stating that the Alabama law conflicts with federal law and undermines federal immigration priorities. The federal lawsuit argues that the state law also expands the opportunities for Alabama police to push immigrants toward incarceration for various new immigration crimes.

The Alabama law, set to take effect September 1, also makes it a crime to knowingly give a ride or provide shelter to an illegal immigrant. It requires schools to report the immigration status of students. Alabama employers also would now be required to check a federal database to determine if new workers are in the country legally.

The Justice Department, in its filing, says a state cannot set its own immigration policy and cannot pass laws that conflict with federal immigration laws.

Already the law is facing mounting opposition.

On Monday, a coalition of religious leaders in Alabama filed suit, challenging the law. The lawsuit by Roman Catholic, United Methodist and Episcopal bishops says the new law "makes it a crime to follow God's command to be Good Samaritans."

Last month, a coalition of civil rights and immigrant rights groups also filed suit, seeking to bar the law from taking effect.

The sponsor of the Alabama law, Republican state Rep. Micky Hammon, defended it Monday.

"The Obama administration and the federal bureaucrats have turned a blind eye toward the immigration issue and refuse to fulfill their constitutional duty to enforce laws already on the books. Now, they want to block our efforts to secure Alabama's borders and prevent our jobs and taxpayer dollars from disappearing into the abyss that illegal immigration causes," Hammon said.

"Allowing hundreds of thousands of illegal immigrants to run unchecked under the radar threatens our homeland security and insults those who come here legally," he added.

Among its provisions, the new law requires the public schools to determine the immigration status of its students and whether they qualify for classes in English as a second language. Opponents contend that provision could have a chilling effect by potentially discouraging parents in the US illegally from enrolling children even if those youngsters are citizens. Officials counter that the state's tough new immigration law won't prohibit any child, illegal immigrant or not, from enrolling in Alabama's public schools.

The Justice Department's complaint quoted Birmingham police chief A.C. Roper as saying the law would divert scarce resources from local policing priorities to immigration enforcement.