Six gay and lesbian couples in the US state of South Dakota have filed a challenge to the state’s same-sex marriage ban.

The move left North Dakota as the only US state which has a ban on same-sex marriage which is unchallenged in court.

The state of South Dakota banned same-sex marriage in 1996 through its Legislature, and then ten years later voters approved a constitutional amendment defining marriage as between one man and one woman.

South Dakota Attorney General Marty Jackley has said he is required by law to defend state laws, and its constitution. He has reiterated that he will do so, should a lawsuit be filed.

Five out of six of the couples bringing the case against South Dakota’s ban have already married in other states which allow same-sex marriage. The sixth was issued a marriage licence on Monday.

Nancy Robrahn, 68, and Jennie Rosenkranz, 72, have been together for 27 years and married in the state of Minnesota last month.

There are now 30 US states with legal challenges against same-sex marriage bans. Another 19, and the District of Columbia allow same-sex couples to marry.

North Dakota remains the only state with no legal challenge against its same-sex marriage ban, but that will change in current weeks, say same-sex marriage advocates in the state.