WASHINGTON (REUTERS) - Virginia will no longer defend its ban on same-sex marriage, the attorney general said on Thursday, making it the latest US state to challenge a prohibition on gay marriage.

Attorney General Mark Herring said Virginia needed to be "on the right side of history" despite a constitutional amendment in the southern state defining marriage as a union between a man and a woman.

"After a very careful and thorough analysis, I believe Virginia's ban on marriage between same-sex couples violates the 14th amendment of the United States Constitution, and as attorney general, I cannot and will not defend laws that violate Virginian's rights," he told National Public Radio.

Virginia's legal turnabout is the latest in a series of state-based challenges on the issue. Federal judges recently overturned such bans in Oklahoma and Utah.

Indiana, meanwhile, is pushing ahead with its own ban.

Mr Herring, a Democrat who took office this month, plans to announce the decision later on Thursday when he asks a federal court to strike down the state's ban, according to The Washington Post.

His office said he will make a statement on the state's position at 11 a.m. EST (1600 GMT).

According to the newspaper, Mr Herring plans to file a brief supporting two same-sex couples who are challenging the ban.

In it, he plans to draw comparisons to another landmark marriage case that stemmed from Virginia decades ago, the Post said, citing a source who has seen Mr Herring's brief. That case, Loving v. Virginia, successfully challenged a ban on interracial marriage.

Virginia's ban on gay and lesbian marriage also denies people the fundamental right to marriage, Mr Herring told NPR.

"The Commonwealth will be siding with the plaintiffs in this case and with every other Virginia couple whose right to marry is being denied," he said.

Seventeen states plus the District of Columbia now recognize gay marriage, including eight states where same-sex marriage became legal in 2013. Thirty-three ban gay couples from marrying by state constitutional amendment, statute, or both.

Virginia's reversal follows two major Supreme Court rulings on the issue last year.

One struck down the Defense of Marriage Act, a law that denied federal benefits to legally married same-sex couples. The other paved the way for gay marriage to resume in California.

But those rulings did not address whether state bans on same-sex marriage are constitutional.

Mr Herring's move underscores the state's political battles and shifting demographics as Democrats try to wrest the once solidly conservative state away from Republicans.

He took office earlier in January along with newly elected Democratic Governor Terry McAuliffe.

Mr Herring's predecessor, Republican Ken Cuccinelli, was a vocal opponent of gay marriage. In 2006, 57 per cent of Virginians voted in favor of the state's marriage ban and Mr Cuccinelli had pledged to uphold it.

While Mr Herring also previously opposed gay and lesbian marriage as a member of the state's legislature, he has since said his views have changed.