Barack Obama has become the first U.S. president to state that "same sex couples should be able to get married," days after his vice president expressed support for the civil rights of gay couples.

Obama made his position clear during an interview with ABC News correspondent Robin Roberts. He said his feelings about the issue are the result of an "evolution" in his thinking, brought about during years of conversations with friends, family, staff members and openly gay service men and women.

"When I think about members of my own staff who are in incredibly committed monogamous relationships, same-sex relationships, who are raising kids together, when I think about those soldiers or airmen or marines or sailors who are out there fighting on my behalf and yet feel constrained, even now that Don't Ask Don't Tell is gone, because they are not able to commit themselves in a marriage, at a certain point I've just concluded that for me, personally, it is important for me to go ahead and affirm that I think same sex couples should be able to get married," Obama said.

Obama went on to say that his comments reflect his own personal opinion, and said he still believes that individual states have the right to decide on the issue for themselves.

Obama's comments are essentially a return to beliefs the president appeared to hold in 1996, when he expressed support for gay marriage in a questionnaire as a state Senate candidate. When he ran for the U.S. Senate in 2004, he seemed to reverse course, saying he believed that marriage is between a man and a woman.

In recent years, however, he has told interviewers that his position was evolving.

The news comes days after Vice President Joe Biden expressed his support for married gay couples receiving the same civil rights and liberties as heterosexual couples, and one day after North Carolina voted to enshrine marriage as between a man and a woman in the state constitution.

The president said Tuesday he was "disappointed" with the vote in North Carolina, a state he won by a narrow margin in the 2008 election.

Some of Obama's core young supporters who helped propel him to the White House on his message of hope and change four years ago had expressed disappointment that the president had not spoken out in favour of same-sex marriage.

A recent Gallup poll suggests the issue is becoming less contentious in the United States, with 50 per cent of respondents saying they are in favour of same-sex marriage. In that survey, 65 per cent of Democrats said they support gay marriage, as did 57 per cent of independents. Seventy-four per cent of Republicans said same-sex marriage should be illegal.

Ten states have bans on same-sex marriage, while six states and the District of Columbia allow gay marriage.

In the interview with ABC, Obama said his daughters, Malia and Sasha, have friends whose parents are in same-sex relationships, and expressed their dismay that they may be treated differently in society than he and the First Lady.

"It's interesting, some of this is also generational," Obama said. "You know, when I go to college campuses, sometimes I talk to college Republicans who think that I have terrible policies on the economy, on foreign policy, but are very clear that when it comes to same sex equality or, you know, sexual orientation, that they believe in equality. They are much more comfortable with it."

Obama said the First Lady shares his opinion, which he acknowledged might appear to put them at odds with their Christian faith.

"When we think about our faith, the thing at root that we think about is, not only Christ sacrificing himself on our behalf, but it's also the Golden Rule, you know, treat others the way you would want to be treated," Obama said. "And I think that's what we try to impart to our kids and that's what motivates me as president, and I figure the most consistent I can be in being true to those precepts, the better I'll be as a dad and a husband and hopefully the better I'll be as president."

The all-but-declared Republican presidential nominee, Mitt Romney, has said he is opposed to same-sex marriage.

With files from The Associated Press

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