New York lawmakers have narrowly voted to legalise same-sex marriage, handing activists a breakthrough victory in the state where the American gay rights movement was born.

New York will become the sixth state where gay couples can wed and the biggest by far.

Gay rights advocates are hoping the vote will galvanise the movement around the US and help it regain momentum after an almost identical bill was defeated here in 2009 and other similar measures failed in recent years.

Though New York is a relative latecomer in allowing gay marriage, it is considered an important prize for advocates, given the state's size and New York City's international stature and its role as the birthplace of the gay rights movement, which is considered to have started with the Stonewall riots in Greenwich Village in 1969.

The New York bill cleared the Republican-controlled state Senate on a 33-29 vote.

The Democrat-led Assembly, which passed a different version last week, is expected to pass the new version with stronger religious exemptions and Democratic Governor Andrew Cuomo, who campaigned on the issue last year, has promised to sign it.

Same-sex couples can begin marrying 30 days after that. The passage of New York's legislation was made possible by two Republican senators who had been undecided.

Senator Stephen Saland pledged the deciding vote, after voting against a similar bill in 2009, helping kill the measure and dealing a blow to the gay rights movement.

"While I understand that my vote will disappoint many, I also know my vote is a vote of conscience," Mr Saland said. "I am doing the right thing in voting to support marriage equality."

New York, the nation's third most populous state, will join Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the Washington capital district in allowing same-sex couples to wed.