WASHINGTON - US President Barack Obama on Wednesday announced his support for same-sex marriage during an interview with ABC News, reversing his long-standing opposition to the matter.

In an interview with ABC News' Robin Roberts, Obama described his thought process as an "evolution" that led him to this decision, based on conversations with his own staff members, openly gay and lesbian service members, and conversations with his wife and own daughters.

"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, in an interview to appear on ABC's "Good Morning America" Thursday. Excerpts of the interview are set to air Wednesday night on ABC's "World News with Diane Sawyer."

The president stressed that this is a personal position, and that he still supports the concept of states deciding the issue on their own. But he said he is confident that more Americans will grow comfortable with gays and lesbians getting married.

Previously, Obama has moved in the direction of supporting same-sex marriage but has consistently stopped short of outright backing it, only voicing support for civil unions for gay and lesbian couples that provide the rights and benefits enjoyed by married couples, though not defined as "marriage."

Obama also opposed efforts to ban gay marriage at the state level, saying that he did not favor attempts to strip rights away from gay and lesbian couples.

Before the president, Vice President Joe Biden said he is "absolutely comfortable" with allowing same-sex couples to wed.