Abortion is again illegal in North Dakota after the state's Supreme Court couldn't muster the required majority to uphold a judge's ruling that struck down the US state's ban last year.

The law makes it a crime for anyone to perform an abortion, though it specifically protects patients from prosecution.

Doctors could be prosecuted and penalised by as much as five years in prison and a $US10,000 ($A15,494) fine.

Three justices agreed the ban is unconstitutionally vague under the state constitution. The other two justices said the law is not unconstitutional.

The state constitution requires at least four of the five justices to agree for a law to be found unconstitutional.

In his opinion, Justice Jerod Tufte said the natural rights guaranteed by the state constitution in 1889 do not ext

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