US government sues Arizona over immigration law
The US government has filed a legal challenge to an Arizona state law targeting illegal immigrants.
The justice department says the law is unconstitutional as it usurps the federal administration's authority to set immigration policy.
The Arizona law, due to take effect on 29 July, makes it a crime to be in the state without immigration papers.
It is opposed by President Barack Obama, civil liberties groups, and by the Mexican government.
The Arizona legislature says it was forced to act because the federal government failed to do so.
The border state is the biggest entry point into the US for illegal immigrants, and is home to an estimated 460,000 illegal immigrants.
Arizona Governor Jan Brewer signed the measure into law in April and it is due to go into effect on 29 July.
But the justice department in Washington, which filed its case in a federal court in Arizona, has asked for implementation of the law to be delayed until the case is resolved.
"In our constitutional system, the federal government has pre-eminent authority to regulate immigration matters," the legal challenge says.
Federal laws do not permit a "patchwork of state and local immigration policies", the justice department added.
A spokeswoman for Ms Brewer described the administration's move as a "terribly bad decision", saying the state had a "terrible border security crisis that needs to be addressed".
Arizona's Republican Senators Jon Kyl and John McCain also criticised the decision to sue their state.
"The Obama administration has not done everything it can do to protect the people of Arizona from the violence and crime illegal immigration brings to our state," they said in a joint statement.
The law was passed after years of frustration in the state over problems associated with illegal immigration, including drug-trafficking and violent kidnappings.
In a speech on immigration reform last week, Mr Obama warned of the possibility of other states following Arizona's lead.
"As other states and localities go their own ways, we face the prospect that different rules for immigration will apply in different parts of the country," he said.
The Arizona law requires state and local officers - who are not federal agents - to query the immigration status of people stopped for a legitimate reason who arouse suspicion of being in the US illegally.