Alabama governor Kay Ivey has signed into law the most restrictive abortion ban in the United States, outlawing nearly all terminations, even in cases of rape and incest.

Under the new measure, performing an abortion will be a crime punishable by a prison sentence of up to 99 years. Abortions would only be legal if the life of the mother is in danger or the foetus has a fatal condition.

The law is now scheduled to take effect in six months, but it is widely expected to be challenged in court.

Republican-led legislatures in state after state are passing bills restricting abortion access in a bid to eventually challenge a 1973 US Supreme Court ruling allowing a woman to terminate a pregnancy.

Opponents of abortion rights are hoping that the moves will prompt the nation's highest court, which now has a conservative majority, to overturn its landmark decision in the case known as Roe vs Wade.

Governor Ivey said that for now, the law may be unenforceable because of Roe vs Wade.

But "the sponsors of this bill believe that it is time, once again, for the US Supreme Court to revisit this important matter, and they believe this act may bring about the best opportunity for this to occur," Ms Ivey said in a statement.

Last week, the Republican governor of Georgia signed into law a ban on abortion from the moment a foetal heartbeat is detected, becoming became the sixth US state to outlaw abortion after roughly six weeks of gestation.

Ohio, Mississippi, Kentucky, Iowa and North Dakota have enacted similar laws, while electoral powerhouses Florida and Texas are considering following suit.

All of the state bans have either been blocked by a judge or are headed for the courts, and some of their backers have said - as Governor Ivey mentioned - that they want the issue to go all the way to the nine-member Supreme Court.

The Republican bid to force a showdown over Roe vs Wade comes as President Donald Trump is ramping up for a 2020 re-election campaign with abortion as a hot-button issue.

The Supreme Court has previously reaffirmed a woman's constitutional right to an abortion, but some anti-abortion activists believe the time may have come to turn the tables.

Since taking office, Mr Trump has appointed two conservative justices - Neil Gorsuch and Brett Kavanaugh - and liberal members of the court are outnumbered five to four.

Conservative-leaning Chief Justice John Roberts is seen as the potential swing vote if the constitutionality of abortion eventually comes before the court.