JEFFERSON CITY, Mo. (AP) - A lawsuit challenging Missouri's ban on gay marriage has been moved to federal court instead of state court.

The lawsuit was originally filed in Jackson County by the American Civil Liberties Union on behalf of two same-sex couples who were denied marriage licenses.

Attorney General Chris Koster's office intervened in the case and moved it to U.S. District Court. Koster's office said federal court is appropriate because the lawsuit alleges that Missouri's gay-marriage prohibition violates the U.S. Constitution.

Missouri voters adopted a state constitutional amendment in 2004 that limits marriage to one man and one woman.

A separate legal challenge still is pending in St. Louis Circuit Court. That case was prompted when St. Louis issued marriage licenses to several gay couples despite Missouri's prohibition.