Maine and Maryland have become the first US states to approve same-sex marriage by popular vote, while Washington and Colorado set up a showdown with federal authorities by legalising recreational use of marijuana.

The outcomes for those ballot measures were a milestone for persistent but often thwarted advocacy groups and activists who for decades have pressed the causes of gay rights and drug decriminalisation.

"Today the state of Washington looked at 70 years of marijuana prohibition and said it's time for a new approach," said Alison Holcomb, manager of the campaign that won passage of Initiative 502 in Washington.

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Colorado governor John Hickenlooper, a Democrat who opposed legalisation, was less enthused.

"Federal law still says marijuana is an illegal drug," he said.

The results in Maine and Maryland broke a 32-state streak, since 1998, in which gay marriage had been rebuffed by every state that voted. They will be the seventh and eighth states to allow same-sex couples to marry.

In Massachusetts, where assisted suicide was on the ballot, backers of a question legalising physician-assisted suicide for the terminally ill conceded defeat, even though the vote was too close to call.

The outcomes of the same-sex marriage votes could influence the US Supreme Court, which will soon consider whether to take up cases challenging the law.