Image caption Governor Jack Dalrymple said the laws aimed to test the boundaries of Roe v Wade

North Dakota has banned abortion once a foetal heartbeat can be detected - as early as six weeks - in the most restrictive law of its kind in the US.

Governor Jack Dalrymple signed a second law banning abortions based on genetic abnormalities.

He approved a third law requiring doctors who perform abortions to have hospital-admitting privileges.

Correspondents say the laws are in part an effort to close the state's only abortion clinic, in the town of Fargo.

The measures, which take effect on 1 August, make no exceptions for rape, incest or the health of the mother.

Gov Dalrymple said: "Although the likelihood of this measure surviving a court challenge remains in question, this bill is nevertheless a legitimate attempt by a state legislature to discover the boundaries of Roe v Wade."

Legal challenge

In that landmark 1973 case, the US Supreme Court ruled abortion was legal until the foetus could survive outside the womb.

Image caption North Dakota's law comes weeks after Arkansas also tightened its abortion rules

The governor said that the court has allowed states to adopt stricter abortion measures, and has never before considered a measure like this one - leaving the constitutionality of the bills an "open question".

Gov Dalrymple added the state should put money aside to pay for legal challenges to the laws.

Under the North Dakota bills, women would not be prosecuted for having an abortion after a foetal heartbeat could be detected, but doctors could face five years in prison and a $5,000 (£3,300) penalty.

Pro-choice advocates vowed to challenge the legislation.

Sarah Stoesz, president of Planned Parenthood, said: "This sweeping package of bills will not stand up to constitutional scrutiny.

"But as a result of North Dakota's leaders' disregard for women's health, the state will endure months and years of drawn-out litigation costing taxpayers hundreds of thousands of dollars."

In addition, North Dakota's Republican-dominated legislature last week set up a voter referendum for November 2014 which seeks to amend the state constitution to define life as beginning at conception.

The amendment would grant full legal protection to embryos and foetuses and could outlaw some forms of birth control, stem-cell research and possibly in vitro fertilisation.

Earlier in March, the Republican-controlled legislature in the state of Arkansas enacted tough abortion laws, banning the procedure after 12 weeks of pregnancy.

Scientists generally agree that foetuses become "viable" or able to survive outside the womb at about 22-24 weeks.