Image copyright AP Image caption Dalrymple, a Republican, signed the strict state anti-abortion bill into law in 2013

A US federal judge has overthrown a North Dakota law banning abortion after the foetus' heartbeat can be detected, as early as six weeks in some cases.

District Judge Daniel Hovland found the law "invalid and unconstitutional" on Wednesday.

The North Dakota abortion law was considered the most restrictive in the nation.

The state attorney general has not yet announced whether he plans to appeal against the decision.

"The United States Supreme Court has spoken and has unequivocally said no state may deprive a woman of the choice to terminate her pregnancy at a point prior to viability," Judge Hovland wrote in his ruling.

'A blatant violation'

The US Supreme Court ruled in the landmark Roe v Wade decision of 1973 that abortion must be legal until a foetus is viable, typically 22-24 weeks.

The strict North Dakota anti-abortion law was one of four such bills Republican Governor Jack Dalrymple signed into law in 2013.

The state's sole abortion clinic and the Center for Reproductive Rights sued to overturn the law last July.

"The court was correct to call this law exactly what it is: a blatant violation of the constitutional guarantees afforded to all women," Center for Reproductive Rights president Nancy Northrup wrote in a statement following the ruling.

State Attorney General Wayne Stenehjem said he would discuss the ruling with Mr Dalrymple before determining his next steps.

"There are those who believed that this was a challenge that could go to the Supreme Court," he told the Associated Press news agency.

"Whether or not that's likely is something we need to confer about."