North Dakota remains as the only state in the US with an unchallenged same-sex marriage ban, but that may all change as a lawsuit is already in the pipeline there.

South Dakota became the latest state with a legal challenge to a ban on same-sex marriage, as six couples filed a lawsuit there at the end of last week.

This left North Dakota as the only state in the US with a law banning gay and lesbian couples from marrying without a legal challenge pending.

But according to reports from the Washington Post, the attorney who filed the South Dakota lawsuit says there are plans to do the same in North Dakota soon.

Joshua Newville said within six to eight weeks, a case will be brought against the ban either by himself or another attorney.

Voters in North Dakota approved a ban on same-sex marriage back in 2004, with 73%.

Same-sex couples can now marry in 19 states across the US, and the District of Columbia.