A lawsuit brought this week by six same-sex couples in South Dakota challenges the state's ban on gay marriage, which makes North Dakota the only state in the nation that has not been sued for prohibiting same-sex unions.

The couples are challenging a law approved by South Dakota lawmakers in 1996 and a 2006 voter-approved constitutional amendment. They also want South Dakota to recognize same-sex marriages performed in states where such weddings are legal.

The lawsuit claims South Dakota's gay marriage ban violates protections guaranteed in the U.S. Constitution: deprivation of equal protection, due process and right to travel, the Associated Press notes.

Among the state officials named as defendants in the suit are Gov. Dennis Daugaard and Attorney General Marty Jackley.

The lawsuit was filed by Minneapolis lawyer Josh Newville, who said he is also interested in challenging the gay marriage ban in North Dakota, where voters in 2004 by a wide margin approved a constitutional prohibition of same-sex marriage.

"Like the rest of the country, South Dakota is a state full of loving and committed couples, many of whom are same-sex, who desire to be united in marriage," said Newville, who graduated from the University of Minnesota Law School in 2012.

Challenges to gay marriage bans are now pending in 30 states, the AP notes.

Same-sex marriage is legal in 19 states, including Minnesota. State legislators made gay marriage legal last year.

A recent poll showed that Minnesotans remain evenly split on the issue.