The administration of U.S. President Barack Obama has filed a legal brief with the Supreme Court to strike down a 1996 law defining marriage exclusively as a union between a man and a woman.
The document, filed Friday, marks the first time a U.S. president has endorsed same-sex marriage rights before the high court.
The White House's position is that the Defense of Marriage Act - DOMA - violates the constitutional guarantee of equal protection under the law.
The brief said DOMA denies federal benefits to tens of thousands of same-sex couples that are granted to heterosexual couples.
The case before the Supreme Court involves Edith Windsor, a lesbian who had been legally married to her decades-long partner. However, Windsor was required to pay a hefty estate tax bill when her partner died because she was not considered legally married under DOMA.
Republicans in both the Senate and the House of Representatives have voiced their opposition to the president's stance on gay marriage.
The Supreme Court is scheduled to hear the case next month.
Also, the court will consider next month a California law, Proposition 8, which banned gay marriage in that state.
Some information for this report was provided by AFP and Reuters.