WASHINGTON (AFP) - The US Supreme Court will rule once and for all on whether gay and lesbian couples have the right to marry after announcing on Friday that it would hear a case to settle the long-running civil rights issue.

In a move that sets the stage for a historic final decision, the court said it would convene in April to study cases in four states - Ohio, Michigan, Tennessee and Kentucky - which have banned gay marriage.

Justices will address two questions: whether states are bound by the Constitution to licence a same-sex marriage, and whether a state is required to recognise same-sex marriages which took place out-of-state.

If they answer yes, same-sex marriage will become legal in all 50 US states.

Attorney General Eric Holder said in a statement that the Justice Department will file a brief in the case "that will urge the Supreme Court to make marriage equality a reality for all Americans."

"It is time for our nation to take another critical step forward to ensure the fundamental equality of all Americans-no matter who they are, where they come from, or whom they love," Holder said.

The Justice Department made the decision to advocate for same-sex marriages several years ago.

The prospect of a final resolution to the thorny question was welcomed by both supporters and opponents of same-sex marriage on Friday.

"Marriage has returned to the US Supreme Court faster than virtually any other issue in American history, and there's a simple reason for that. Committed and loving gay and lesbian couples, their children, and the fair-minded American people refuse to wait a single day longer," said Human Rights Campaign president Chad Griffin.

"We've reached the moment of truth ... now the nine justices of the Supreme Court have an urgent opportunity to guarantee fairness for countless families, once and for all."

Groups against same-sex marriage also praised the Supreme Court's decision to take up the case.

"The US Supreme Court now has the opportunity to issue a long-overdue ruling to restore the freedom of the people to uphold marriage in their state laws as the union of a man and a woman," Family Research Council president Tony Perkins said.

Brian Brown, president of the National Organisation for Marriage agreed, saying the "cherished institution of marriage which forms the very bedrock of our society."

Meanwhile celebrities also saluted the prospect of a day in court.

"Gay marriage is headed to the SUPREME COURT. Lets get ready to rumble," pop singer Lady Gaga wrote on Twitter.

"Raise your voices, share your stories. You deserve justice. #equality."

- Preceding ruling -

In a landmark decision in June 2013, the Supreme Court struck down a law denying federal benefits to homosexual couples by defining marriage as a union between a man and a woman.

In doing so, it cleared the way for married gay and lesbian couples to enjoy the same rights and privileges under federal law as their straight counterparts.

But it stopped short of legalising same-sex marriage nationwide, leaving that question to the states.

At that time, only 12 states plus the capital Washington recognised same-sex marriages.

Since then, a patchwork legalisation of same-sex marriage has rolled out across America, with 36 states now allowing it.

Same-sex unions in the state of Florida - with its huge swath of southern rural conservatism juxtaposed by large areas of urban liberalism - met with years of resistance before finally being allowed earlier this month.

With a few exceptions, the states that have refused to marry same-sex couples have lost in court, including in front of the Supreme Court.

The question has been up in the air since Massachusetts became the first state to legalise same-sex marriage in 2004.

Lawyers will brief the court on the marriage cases between February and April, the Supreme Court said in its order taking up the issue.