WASHINGTON- The U.S. Supreme Court on Monday declined to hear a challenge by a men's rights group to the national requirement that men, but not women, register for the military draft at age 18, focusing on whether the policy violates the U.S. Constitution's guarantee that laws apply equally to everyone.

The justices turned away an appeal by the National Coalition for Men, backed by the American Civil Liberties Union, of a lower court ruling upholding the policy. The lawsuit was aimed at invalidating a 1948 U.S. law that compels only men to register for potential military service.

Liberal Justice Sonia Sotomayor issued a statement noting that a commission set up by Congress last year recommended ending the male-only draft and that Congress could act on it.

Sotomayor wrote that the court's "longstanding deference to Congress on matters of national defense and military affairs cautions against granting review while Congress actively weighs the issue."

Liberal Justice Stephen Breyer and conservative Justice Brett Kavanaugh signed on to Sotomayor's opinion.

By rebuffing the appeal, the justices left in place the lower court ruling and a 1981 Supreme Court precedent that had upheld the law in part because women at that time were prohibited from serving in combat roles in the armed forces.

The Pentagon in 2013 lifted the official ban on women in combat. ACLU lawyers had argued that it made little sense that in light of that policy shift women were not required to register for the draft alongside men.

(Reporting by Lawrence Hurley; Additional reporting by Andrew Chung; Editing by Will Dunham) ((lawrence.hurley@thomsonreuters.com; Twitter: @lawrencehurley; +1 202-809-3080))