WASHINGTON (Reuters) - The U.S. Supreme Court on Monday halted same-sex marriages in Utah - permitted for less than three weeks - while the state appeals a lower-court ruling legalizing them, as the justices again left their imprint on the debate over gay nuptials.

The high court granted a request from state officials appealing a federal judge's Dec. 20 ruling that had allowed same-sex weddings to go ahead in the heavily Mormon state.

The action by the court means that gay weddings in the state are on hold for now while the case is appealed to the Denver-based 10th U.S. Circuit Court of Appeals. Hundreds of gay couples in Utah have received marriage licenses since the ruling by U.S. District Judge Robert Shelby.

The stay created uncertainty about the legal status of the roughly 1,000 same-sex marriages that have taken place so far.

Sean Reyes, Utah's attorney general, told reporters in Salt Lake City that the state is not yet certain whether the stay invalidates existing marriages.

"This is precisely the uncertainty that we were hoping to avoid," Reyes said. "It's unfortunate that many Utah citizens have been put into this legal limbo."

Gary Herbert, Utah's Republican governor, said Shelby should have stayed his ruling, as officials had requested. The appeals court also declined to stay the decision, which prompted the state's emergency application to the high court.

"I firmly believe this is a state-rights issue and I will work to defend the position of the people of Utah and our state constitution," Herbert said in a statement.

The Supreme Court's action on Monday was only on the matter of whether there should be a stay of Shelby's ruling. The high court's order was two sentences long, with no justices writing individual opinions indicating where they might stand on the merits of the case.

But the fact the court granted the stay was enough to put the spotlight on where the justices stand on gay marriage six months after their two high-profile decisions on the matter.

One ruling struck down the Defense of Marriage Act (DOMA), a federal law that denied federal benefits to legally married same-sex couples. The other paved the way for gay marriage to resume in California.

In both cases, the court avoided making any sweeping pronouncements about a right to gay marriage in the United States.

The appeals court has already agreed to hear the Utah case on an expedited schedule, with a Feb. 25 deadline for court papers.

Terry Henry, a Utah special education teacher who took advantage of Shelby's decision to marry Penny Kirby, said she was surprised by the decision to grant a stay.

"The world seems a little less stable today," Henry said.

Henry, 47, added that she wonders what impact the stay might have on her newly minted family's rights. Having a marriage license allowed Henry to add the unemployed Kirby, 51, to her work-based health insurance just last week.

'AN AFFRONT'

Brian Brown, president of the National Organization for Marriage, which opposes gay marriage, characterized Monday's high court action as a repudiation of Shelby's decision.

"The actions of this activist judge are an affront to the rule of law and the sovereign rights of the people of Utah to define marriage," he said in a statement.

Gay marriage supporters including the American Civil Liberties Union sought to downplay the high court's action.

"Despite today's decision, we are hopeful that the lower court's well-reasoned decision will be upheld in the end and that courts across the country will continue to recognize that all couples should have the freedom to marry," ACLU lawyer Joshua Block said in a statement.

Utah become the 18th state - at least temporarily - where gay marriage was permitted when Shelby sided with three same-sex couples in their lawsuit challenging a voter-passed amendment to the Utah state constitution that defined marriage as exclusively between a man and a woman.

Little more than a decade ago, none of the 50 U.S. states recognized same-sex marriage. Since then, attitudes have changed rapidly in some parts of the country.

At the time of the Supreme Court rulings in June, only 12 states and the District of Columbia recognized gay marriage. Since then, more states have followed, some via legislative action and others due to court rulings. Hawaii, Illinois and New Mexico are the most recent states where gay marriage has become legal.

Shelby's decision came as a shock to many of Utah's 2.8 million residents, nearly two-thirds of whom are members of the Church of Jesus Christ of Latter-day Saints. Mormon doctrine states that sexual relations outside opposite-sex marriage are contrary to the will of God.

Utah's stay application relied in part on the high court's June decision in United States v. Windsor, which, although it struck down DOMA, also said the definition of marriage was largely a matter of state law.

In the Windsor case, in which the court was split 5-4, Justice Anthony Kennedy wrote for the majority that the federal law violated the U.S. Constitution's guarantee of equal protection.

(Reporting by Lawrence Hurley; Additional reporting by Jennifer Dobner in Salt Lake City; Editing by Will Dunham)