(Reuters) - The nation's largest civil rights group, the NAACP, endorsed gay marriage on Saturday, giving a boost to the movement to legalize same-sex nuptials despite reservations expressed by some black ministers.

The National Association for the Advancement of Colored People passed the resolution at its board meeting in Miami "as a continuation of its historic commitment to equal protection under the law," the organization said.

"Civil marriage is a civil right and a matter of civil law. The NAACP's support for marriage equality is deeply rooted in the Fourteenth Amendment of the United States Constitution and equal protection of all people," Benjamin Todd Jealous, President and CEO of the NAACP, said in a statement.

The gay rights movement got a big boost last week when President Barack Obama said for the first time he supports gay marriage.

After he announced his decision, Obama held a conference call with at least eight black ministers, some of whom were skeptical, to explain his position, The New York Times reported.

Three state legislatures have voted this year to legalize gay marriage - New Jersey, Maryland and Washington state - although New Jersey Republican Governor Chris Christie vetoed the measure. In Maryland the law passed even though some black Democrats opposed it.

Six other states - New York, Connecticut, Vermont, New Hampshire, Massachusetts, and Iowa plus the District of Columbia - have legalized gay marriage, and a handful of others recognize so-called "civil unions."

"EQUAL MEMBERS OF SOCIETY"

There has been a steady increase in support for same-sex marriage. The Gallup polling organization said in a recent survey that 50 percent of American adults now are in favor of it.

"Americans' acceptance of gays and lesbians as equal members of society has increased steadily in the past decade to the point that half or more now agree ... that gay or lesbian couples should have the right to legally marry," Gallup said on its website.

Church-going black Americans are divided over gay marriage, according to a recent Reuters/Ipsos poll, but are unlikely to vote for Republican Mitt Romney over Democrat Obama, the nation's first black president, in the November general election.

Romney opposes gay marriage, a position essential for winning the majority of evangelicals.

The NAACP opposed a constitutional amendment approved in North Carolina last week that would ban same sex marriage. Twenty-eight other states have voter-approved constitutional bans on same-sex marriages, according to the National Conference of State Legislatures.

The NAACP resolution was welcomed by gay rights group Human Rights Campaign.

"We could not be more pleased with the NAACP's history-making vote," Human Rights Campaign President Joe Solmonese said. "It's time the shameful myth that the African-American community is somehow out of lockstep with the rest of the country on marriage equality is retired - once and for all. The facts and clear momentum toward marriage speak for themselves."

(Reporting by Mary Slosson and Greg McCune; Editing by Cynthia Johnston and Xavier Briand)