President Donald Trump on Saturday declared himself "strongly Pro-Life" but in favor of exceptions for pregnancies resulting from rape or incest, after several US states passed tough new restrictions on abortions.

The US president spelled out his position on abortion, set to be a hot-button issue at next year's election, days after Alabama's governor signed the country's most restrictive law - enacting a near-total prohibition even in cases of rape and incest.

"As most people know, and for those who would like to know, I am strongly Pro-Life, with the three exceptions - Rape, Incest and protecting the Life of the mother - the same position taken by Ronald Reagan," tweeted Trump.

Trump, a bombastic, twice-divorced billionaire, won over the evangelical vote during his 2016 campaign by promising to appoint anti-abortion justices at the Supreme Court.

He has since brought two conservative appointees to the highest court in the land - Neil Gorsuch and Brett Kavanaugh - shifting the balance of the nine-person bench.

American evangelicals now have high hopes that the court will chip away at its historic 1973 US decision to legalize abortion.

In addition to Alabama, the Missouri legislature this week made abortions illegal from eight weeks of pregnancy.

Georgia, Ohio, Mississippi, Kentucky, Iowa and North Dakota have enacted laws banning abortion from the moment a fetal heartbeat is detected.