North Carolina voters have approved a state constitutional amendment that bans same-sex marriage and civil unions, dealing a blow to efforts across the United States to extend gay marriage rights.

The amendment, which defines marriage between a man and a woman as the only legally recognised domestic union in the state, was passed by a wide margin on Tuesday.

With 95 of 100 counties' results reported, about 61 per cent of voters backed the amendment.

North Carolina law already blocks gay and lesbian couples from marrying, but the state now joins other southeastern states in adding the prohibition to its constitution.

Many voters simply viewed the amendment as a vote on same-sex marriage despite efforts by the measure's opponents to broaden the discussion, Tom Jensen of the Raleigh-based Public Policy Polling firm said.

"Anytime in North Carolina you have a majority of Republicans and African Americans on the same side of an issue, that's a very potent combination," Jensen said.

Gay rights struggle

Twenty-eight other states have voter-approved constitutional bans on same-sex marriages, according to the National Conference of State Legislatures.

Massachusetts, Iowa, Vermont, New Hampshire, Connecticut, New York and the District of Columbia allow gay and lesbian nuptials.

The vote in North Carolina followed statements by senior officials of President Barack Obama's administration this week which were interpreted as supporting gay marriage.Maryland, New Jersey and Washington state have passed laws this year approving same-sex marriage, but Governor Chris Christie vetoed New Jersey's law and opponents in Maryland and Washington are threatening ballot initiatives to overturn those laws.

Vice President Joe Biden said on Sunday he was "absolutely comfortable" with allowing same-sex couples to wed, and Arne Duncan, the education secretary, said gay marriage should be legal.

Obama has said he favours civil unions but has stopped short of supporting gay marriage.

Supporters of the amendment in North Carolina, a swing state in the November 6 presidential election, said it would preserve the sanctity of marriage between a man and a woman and make laws forbidding gay marriage harder to repeal.

Opponents said a ban would jeopardise health insurance benefits for unmarried gay and heterosexual couples and signal that the state was unfriendly to a diverse workforce.

Billy Graham, a prominent Christian evangelist, called on voters to support the measure, while Bill Clinton, a former Democratic president, and some business leaders urged North Carolinians to reject it.

"We will not allow marriage to be redefined in this state. The nation is watching North Carolina, and we have given them a high standard to follow," Tami Fitzgerald, chairwoman of the pro-amendment group Vote for Marriage NC, told supporters at a celebration party.