WASHINGTON — The Supreme Court will take up an appeal from Arizona over its requirement that people prove they are American citizens before registering to vote.

The justices on Monday said they will review a federal appeals court ruling that blocked the law in some instances.

A 12-member panel of the 9th U.S. Circuit Court of Appeals in San Francisco said that federal law trumps the Arizona requirement. Federal law allows voters to fill out a mail-in voter registration card and swear they are citizens under penalty of perjury, but it doesn’t require them to show proof as Arizona’s 2004 law does.

The ruling applies only to people who seek to register using the federal mail-in form. Arizona has its own form and an online system to register when renewing a driver’s license. The court ruling did not affect proof of citizenship requirements using the state forms.

Arizona officials have said most people use those methods and the state form is what county officials give people to use to register. But voting rights advocates had hoped the 9th Circuit decision would make the federal mail-in card more popular because it’s more convenient than mailing in a state form with a photocopy of proof of citizenship.

The citizenship requirement stems from Proposition 200, approved by Arizona voters in 2004. The law also denied some government benefits to illegal immigrants and required Arizonans to show identification before voting. The 9th Circuit upheld the voter identification provision. The denial of benefits was not challenged.