By Lawrence Hurley and Jonathan Stempel

WASHINGTON, Feb 19 (Reuters) - The U.S. Supreme Court sided with an American father on Tuesday in an international custody dispute over a girl who has been living outside the country under a lower court order.

By a 9-0 vote, the court ruled that U.S. Army Sergeant Jeffrey Chafin had a right to seek custody of his 6-year-old daughter, Eris, even though she had been taken to Scotland by her mother, Lynne Chafin, a Scottish national.

A lawyer for the mother said no Scottish court would allow the child to return to the United States.

The 11th U.S. Circuit Court of Appeals had dismissed Jeffrey Chafin's appeal, saying the issue was moot because the girl by then was already in Scotland and beyond its control.

The Supreme Court ruled on Tuesday that the appeals court should have considered the merits of the appeal by Jeffrey Chafin, who had filed in 2010 for divorce from his wife.

"This dispute is very much alive," and the Chafins "continue to vigorously contest the question of where their daughter will be raised," Chief Justice John Roberts wrote for the court.

But Roberts said Jeffrey Chafin still faces an uphill battle, noting the possibility that Scottish courts could ignore a U.S. court ruling in his favor.

Michael Manely, a lawyer for Jeffrey Chafin, did not immediately respond to a request for comment.

Stephen Cullen, a lawyer for Lynne Chafin, said his client has a "very very strong case" should the matter return to the 11th Circuit, and predicted Scottish courts would have the ultimate say.

"A Scottish court is never going to return the child to the United States after this amount of time, because it couldn't possibly be good for the child," Cullen said in a telephone interview. "It would be complete madness for the Scottish court to say, OK, it's three years later, we'll send the child back."

Lynne Chafin and Eris now live in Ayrshire, southwest of Glasgow, Cullen said.

In a concurring opinion, U.S. Supreme Court Justice Ruth Bader Ginsburg noted that the legal confusion arose because the U.S. Congress did not indicate how custody appeals should be handled under the Hague Convention on the Civil Aspects of International Child Adoption.

Dragging out the Chafin case "is hardly consistent with the convention's objectives," Ginsburg wrote.

Her opinion was joined by Justices Antonin Scalia and Stephen Breyer.

Lynne Chafin had lived with her daughter in Scotland since 2007, apart from her husband because of his job.

In February 2010, she had traveled with her daughter to visit him in Madison, Alabama, in a failed effort to save their marriage, and was deported after overstaying her visa.

Eris at first stayed behind, but in 2011 a federal judge in Alabama said Lynne Chafin could take her back to Scotland, calling it the child's "habitual residence" under the Hague Convention. Lynne Chafin then took her daughter to Scotland, and the 11th Circuit later dismissed the father's appeal.

The case is Chafin v. Chafin, U.S. Supreme Court, No. 11-1347.

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