NEW ORLEANS (Reuters) - A Louisiana state judge ruled on Monday in favor of a same-sex couple, saying the state's ban on gay marriage is unconstitutional, advocates said.

The ruling compels the state to recognize the out-of-state marriage of the plaintiffs in the case but its full extent is not immediately clear because it is temporarily under seal, said Bruce Parker, a spokesman for Equality Louisiana.

"This judgment of the court is a sign of hope amid the maelstrom of despair and confusion that Louisiana's LGBT community has lived through recently," C. Welton Gaddy, president of Interfaith Alliance and a Monroe-based pastor, said in a statement.

Louisiana Attorney General Buddy Caldwell plans to appeal the decision directly to the state's Supreme Court, the Times-Picayune newspaper reported. His office did not immediately return messages seeking comment.

The ruling comes as the U.S. Supreme Court is expected to take up the question of gay marriage bans during its next term, which starts in October, in what is likely to be the most momentous civil rights case in years.

Earlier this month, a federal judge in New Orleans ruled in favor of the state's same-sex marriage ban, in a break from other recent federal rulings on the issue.

Nineteen U.S. states and the District of Columbia allow same-sex marriage.

Monday's Louisiana case was brought by a lesbian couple seeking to have their California marriage recognized and for the legal adoption of the son they have raised together, Parker said.

The ruling is under seal because of privacy rules governing adoption cases but is expected to be made public on Tuesday with the plaintiffs' consent, Parker said.

An attorney for the plaintiffs did not immediately return messages seeking comment.

(Editing by Sandra Maler)