WASHINGTON (AP) - The US Supreme Court will begin hearing highly-anticipated gay marriage cases this week, and its decisions could redefine marriage in the 40 states that don't allow same-sex couples to wed.

The high court can choose from a wide array of outcomes in the cases of California's voter-approved ban on same-sex marriage and the Clinton-era federal Defence of Marriage Act. The federal law defines marriage as the union of a man and a woman and therefore keeps legally married gay Americans from collecting a range of federal benefits that generally are available to married people.

The cases will be argued on Tuesday and Wednesday; rulings are not likely before late June.

The justices might come out with rulings that are simple, clear and dramatic. Or they might opt for something narrow and legalistic.

The court could strike down dozens of state laws that limit marriage to heterosexual couples, but it also could uphold gay marriage bans or say nothing meaningful about the issue at all.

If the court upholds California's ban, that would leave gay couples without the right to marry in the state and would tell the roughly 40 states that do not allow same-sex marriages that there is no constitutional problem in limiting marriage to a man and a woman.

Such an outcome probably would trigger a political campaign in California to repeal the ban through a ballot measure and could give impetus to similar voter or legislative efforts in other states.

If the court strikes down California's ban, the broadest possible outcome would in effect invalidate constitutional provisions or statutes against gay marriage across the US.

Or a majority of the justices could agree on a middle option that applies only to California as well as Colorado, Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon, and Rhode Island. Those states already treat gay and straight couples the same in almost every respect through civil unions or domestic partnerships.

This "nine-state solution" would say that the Constitution forbids states to withhold marriage from same-sex couples while giving them all the basic rights of married people. But this ruling would not implicate gay marriage bans in other states and would leave open the question of whether states could deprive gay couples of any rights at all.

The narrowest of these potential outcomes would apply to California only. The justices essentially would adopt the rationale of the federal appeals court that found that California could not take away the right to marry that had been granted by the state Supreme Court in 2008 before voters approved the ban later that year.

In addition, if the Supreme Court were to rule that gays and lesbians are deserving of special protection from discriminatory laws, it is unlikely that any state ban on same-sex marriage could survive long, even if the justices don't issue an especially broad ruling in this case.

As for the federal Defence of Marriage Act, a ruling upholding it would not affect state laws regarding marriage but would keep in place federal statutes and rules that prevent legally married gay Americans from receiving a range of benefits that are otherwise available to married people. These benefits include breaks on estate taxes, health insurance for spouses of federal workers and Social Security survivor benefits.

A ruling against the federal law would allow legally married gay couples or, in some cases, a surviving spouse in a same-sex marriage, to receive benefits and tax breaks resulting from more than 1,000 federal statutes in which marital status is relevant. For 83-year-old Edith Windsor, a New York widow whose case is before the court, such a ruling would give her a refund of US$363,000 (S$451,298) in estate taxes that were paid after the death of her spouse, Thea Spyer.

If the Supreme Court finds that it does not have the authority to hear the case, Ms Windsor probably would still get her refund because she won in the lower courts, but there would be no definitive decision about the law from the highest US court and it would remain on the books.

Former President Bill Clinton, who signed the federal law in 1996, recently called for it to be overturned. He said he signed it to avoid legislation that would have been even worse for gays, but now realises the law discriminates against gays and is incompatible with the Constitution.

The Obama administration has stopped defending the law in court.