WASHINGTON - The Obama administration is asking the Supreme Court to declare unconstitutional a section of federal law that recognizes only male-female marriages.

In a filing with the court, the administration says Section 3 of the Defense of Marriage Act denies legally married same-sex couples many federal benefits that are available only to legally married heterosexual couples. Federal tax and Social Security survivor's benefits are among them.

In its brief, the administration said the provision "violates the fundamental constitutional guarantee of equal protection."

The brief was filed as the justices prepare to hear arguments next month in a challenge to the 1996 law.

President Obama last year said he personally supports gay marriage. He said in last month's inaugural address that the law should treat gay people like anyone else.

U.S. Solicitor General Donald Verrilli's legal brief urges the court to uphold a federal appeals court ruling that found the Defense of Marriage Act unconstitutional. The administration's position was not unexpected because it has already expressed its legal position in challenges to DOMA, including in a case involving a San Francisco couple.

But the legal brief marked the first time a president has endorsed same-sex marriage rights in the Supreme Court. Administration lawyers expressed particular concern about federal law denying equal benefits to couples legally married in states that permit same-sex nuptials, including New York, where the case before the Supreme Court has unfolded.

"The law denies to tens of thousands of same-sex couples who are legally married under state law an array of important federal benefits that are available to legally married opposite-sex couples," the solicitor general wrote.

The brief, filed Friday, did not address the separate legal challenge to Proposition 8, California's voter-approved ban on same-sex marriage. The administration is expected by late next week to decide whether to take a position in the California case, which is to be heard in the Supreme Court on March 26.