CHICAGO (AFP) - Minnesota on Monday became the 12th state, and the third in two weeks, to legalise same-sex marriage, marking another victory for marriage equality campaigners in the United States.

Senators in the state capital Saint Paul approved the measure by a 37-30 vote, after it cleared the House of Representatives on Thursday, and Governor Mark Dayton is expected to sign it into law as early as Tuesday.

"I am proud to be a Minnesotan today," said Democratic state senator Scott Dibble during a four-hour debate in which he announced he'll marry his longtime gay partner on August 1, the day the law comes into force.

Rhode Island legalized same-sex marriage on May 2, followed by Delaware last Tuesday. Besides Minnesota, the only Midwestern state to recognise gay marriage is Iowa.

Constitutional bans remain in place in 31 states, while the US Supreme Court is expected to rule in June on the Defense of Marriage Act, under which the federal government recognises heterosexual marriages only.