The US Supreme Court has struck down parts of a controversial immigration law in the state of Arizona, but upheld one key provision allowing police officers to check the immigration status of anyone they stop.

In a split decision, the justices ruled that three provisions of the law went too far, including one that makes it a crime for illegal immigrants to work and another that requires them to carry their documents.

The decision did uphold the "show me your papers" clause by a unanimous decision, but took the teeth out of it by preventing police officers from arresting people on minor immigration charges.

Arizona, on the southwest border with Mexico, two years ago became the first of a handful of US states to pass laws aimed at driving illegal immigrants out, including requiring police to check the immigration status of anyone detained and suspected of being in the country illegally.

The battle over the law goes to the heart of a fierce national debate between Democrats and Republicans over what to do with the roughly 11 million illegal immigrants in the country.

Critics have said the Arizona law could lead to ethnic and racial profiling of the fast-growing Hispanic population in the United States. Hispanics are the largest US minority group.

President Barack Obama said he was pleased that the court struck down most of the law, but "remained concerned" about the provision upheld by the court.

"No American should ever live under a cloud of suspicion just because of what they look like," he said. "Going forward, we must ensure that Arizona law enforcement officials do not enforce this law in a manner that undermines the civil rights of Americans."

Jan Brewer, the governor of Arizona, put her own positive spin on the ruling, calling it a "victory for the rule of law" in a statement on Monday.

"It is also a victory for the 10th Amendment, and all Americans who believe in the inherent right and responsibility of states to defend their citizens," she said, referring to the provision in the US Constitution which delegates powers from the federal government to the states.

An election-year issue

The ruling weakens a drive by Republicans, especially the most conservative in the party, who want to make it more difficult for immigrants who are in the country illegally.

Mitt Romney, the Republican presidential candidate, took a tough stand on immigration in the early days of his campaign for the nomination, but has recently sought a more moderate position. Romney used the ruling to criticise President Barack Obama's immigration policy.

"Today's decision underscores the need for a president who will lead on this critical issue and work in a bipartisan fashion to pursue a national immigration strategy," he said in a statement. "President Obama has failed to provide any leadership on immigration."

Democratic lawmakers were quick to praise the court's ruling. Chuck Schumer, the senior senator from New York, called it "as strong a repudiation of the Arizona law" as possible. Harry Reid, the Senate majority leader, said he worried that the one remaining provision would lead to racial profiling.

"Ultimately, responsibility to fix the nation's broken immigration system lies with Congress," he said. "Republicans must join Democrats to forge fair, tough, practical solutions."

Separately, the court was widely expected to rule on Obama's landmark health care legislation, passed in 2010, but said that ruling would not be issued until Thursday.

The court also reaffirmed a two-year-old decision which guaranteed corporations and labour unions the right to spend unlimited amounts of money on presidential and congressional elections. The justices struck down a law passed in the state of Montana which limited corporate spending.