The US justice department is suing the state of Arizona over a controversial new immigration law.

The law empowers state police to question anyone if they have "reasonable suspicion" that the person is an illegal immigrant.

Critics say the law is unconstitutional and will sap enforcement resources in trying to stem the flood of thousands of illegal immigrants trying to cross the border from Mexico, and to cut down on drug trafficking and other crimes.

The justice department suit argues that with 10.8 million illegal immigrants believed to be in the country, the constitution gives the federal government sole authority over immigration matters, and not individual states.

The suit is asking a federal court in Arizona to grant an injunction to prevent the state law from taking effect on July 29.

Al Jazeera's Rosalind Jordan reports.