WASHINGTON (AFP) - An appeals court in the United States for the first time upheld a ban on same-sex marriage on Thursday, bucking a trend and likely sending the issue back to the nation's highest court.

The decision by the Court of Appeals for the Sixth Circuit determined the US Constitution did not prohibit states from defining marriage as between one man and one woman.

The appeals court covers four states in the middle of the country: Michigan, Ohio, Tennessee and Kentucky.

The decision runs counter to a trend of US courts on all levels ruling in favour of same-sex marriages.

Last month, the US government said it recognised same-sex marriage in six new states, increasing the total to 32 and the District of Columbia.

Thursday's ruling against same-sex marriage will almost surely be petitioned to the Supreme Court, which recently decided not to consider other same-sex cases.

In the 2-1 decision handed down on Thursday, the court ruled that while the definition of marriage was clearly changing in America, court rulings weren't the appropriate way to make the change.

America's federal system makes states to be "laboratories of experimentation" allowing "one State to innovate one way, another State another," the ruling said.

The court suggested that state-focused system made democratic processes such as voting the best method for change.

Direct democracy allows respect for gay individuals and for opponents of gay marriage, the court said.

"When the courts do not let the people resolve new social issues like this one, they perpetuate the idea that the heroes in these change events are judges and lawyers," the ruling said.