After years of "evolving" on the issue, US President Barack Obama said on Wednesday he believes same-sex couples should be allowed to marry, a stance that is likely to please his political base and complicate efforts to attract some independent voters.

"It is important for me to go ahead and affirm that I think same-sex couples should be able to get married," Obama said in an interview with ABC's Robin Roberts.

Obama's comments marked the first time a US president had publicly expressed support for gay marriage, and his position was hailed by Democrats, gay rights groups and others as a benchmark for civil rights in the US.

"This is a major turning point in the history of American civil rights" said New York Mayor Michael Bloomberg, an independent whose city is in one of six states that allow same-sex marriage.

Neera Tanden from the Center for American Progress described the president's expression of support as "another large step toward realizing this country's promise of equality."

Others, including Republican activists and conservative Christian leaders, criticised Obama's stance and called it a huge political risk on a divisive issue.

Some said it could lead independent voters who oppose gay marriage to support Mitt Romney, Obama's likely Republican opponent in the Nov. 6 election. Noting that 29 states have approved bans on same-sex marriage, they said Obama's announcement also could help Romney consolidate support among evangelical Christians who, like Romney, oppose gay marriage.

"Today's announcement ensures that marriage will again be a major issue in the presidential election," said Tony Perkins, a prominent evangelical leader and president of the Family Research Council, a conservative Christian group. "The president has provided a clear contrast between him and ... Mitt Romney."     

Added Ralph Reed of the Faith and Freedom Coalition: "This is an unanticipated gift to the Romney campaign. It is certain to fuel a record turnout of voters of faith to the polls this November."     

Romney, campaigning in Oklahoma City, said he believes "marriage is a relationship between a man and a woman." He has said he supports hospital visitation rights and other domestic partnership benefits for gay and lesbian couples.

Analysts saw Obama's move as a calculated risk at a time when polls indicated that an increasing number of Americans support legally recognizing gay marriages.

A recent Reuters/Ipsos poll said that more than 39 percent of Americans believe same-sex couples should be allowed to marry legally. Another 23.5 percent said that such couples should be allowed to form civil unions but not marry, while nearly 27 percent opposed marriage or civil unions for gay and lesbian couples.

"It is not without political risk," Democratic strategist Julian Epstein said on MSNBC. "Polls show that nearly every segment of the population" is moving toward acceptance of gay marriage, but "Republicans certainly will try to use it as a wedge in the African American community and with non-college educated white voters," key voting blocs in which many people oppose gay marriage. - Reuters