Voters in Maryland and Maine have approved same-sex marriage, while a similar measure in Washington state appears on track to pass.

It is the first time marriage rights have been extended to same-sex couples by popular vote in the US.

The vote was hailed as a watershed moment by gay rights activists because, while same-sex unions have been legalised in six states and the District of Columbia by lawmakers or courts, voters had consistently rejected doing so.

Voters in more than 30 states have approved constitutional bans on gay marriage.

"It's enormous. We have truly made history," said Brian Ellner, head of the pro-gay marriage group The Four.

"Having the first states approve marriage by a popular vote changes the narrative and sends an important message to the Supreme Court."

President Barack Obama earlier this year became the first US president to support gay marriage, and his campaign endorsed the gay marriage measures in the three states.

In Maryland, the measure passed 52% to 48%.

In Maine, it was leading by 53% to 47%, with 75% of precincts reporting.

The six-point margin was wide enough to allow proponents to declare victory.

And in Washington, it was leading by 52% to 48%, with 62% of precincts reporting.

In Minnesota, meanwhile, voters rejected adding that state to the list of those defining marriage solely as a heterosexual union.

With more than 97% of precincts reporting, the proposed constitutional amendment was trailing 48% to 52%.

The constitutionality of restricting marriage to unions between a man and a woman is widely expected to be taken up by the US Supreme Court soon.

James Esseks, director of the ACLU Lesbian Gay Bisexual and Transgender Project, called the votes a "watershed moment" for gay and lesbian families.

"Not long ago, marriage for same-sex couples was unimaginable," he said.

"In a remarkably short time, we have seen courts start to rule in favour of the freedom to marry, then legislatures affirm it, and now the people vote for it as well."

Among the states that have expanded marriage rights to include same-sex couples, in Massachusetts, Iowa and Connecticut the laws followed court rulings that same-sex couples could not be denied marriage rights.

Legislatures approved the change in Vermont, New York and New Hampshire.

Before this year, ballot initiatives banning the legal recognition of same-sex marriage had succeeded in 31 states, and no state had ever approved same-sex marriage by popular vote.

Maine voters rejected gay marriage in a referendum in 2009 by 53 to 47%.

In Washington and Maryland, where state legislatures previously passed laws expanding marriage rights to gay and lesbian couples, it was up to citizens to decide whether to let the laws stand.

"Over these past few weeks, Marylanders joined together to affirm that for a free and diverse people of many faiths - a people committed to religious freedom - the way forward is always found through greater respect for the equal rights and human dignity of all," Governor Martin O'Malley said in a statement.