The governor of South Dakota signed legislation Monday banning almost all abortions in the state, in a direct challenge to the U.S. Supreme Court's decision to legalize the practice more than 30 years ago.

The law makes it criminal for doctors to perform an abortion even in cases that a woman became pregnant as a result of rape or incest.

However, there is a narrow exemption in some cases in which the procedure was necessary to save the pregnant woman's life.

Gov. Mike Rounds issued a written statement saying he expects the legislation to spark a legal battle that will take years to resolve, and will not take effect unless the nation's top court upholds it.

"In the history of the world, the true test of a civilization is how well people treat the most vulnerable and most helpless in their society. The sponsors and supporters of this bill believe that abortion is wrong because unborn children are the most vulnerable and most helpless persons in our society. I agree with them," Rounds said in the statement.

Last month, the Legislature passed the bill after supporters argued that the recent appointment of conservative justices to the high court could weaken or dismantle the landmark Roe v. Wade ruling of 1973.

Under the new law, which will go into effect July 1, doctors could get up to five years in prison and a $5,000 US fine for performing an illegal abortion.

Rounds pointed out that the legislation was written to ensure existing restrictions would still be enforced during the legal battle that is expected to be launched.

Current state law sets increasingly stringent restrictions on abortions as a woman's pregnancy progresses.

After the 24th week, abortions are permitted only to protect the woman's health and safety.

Planned Parenthood, which operates the state's sole abortion clinic, where about 800 procedures are performed each year, has pledged to challenge the new law.

However, the organization has not yet decided whether it will challenge the measure in court, or put the matter up for a statewide public vote in November.

A referendum would either revoke the abortion ban or postpone a court challenge.

Abortion foes have already begun to offer money to help the state pay legal bills for the anticipated court challenge, Rounds said.

One anonymous donor has pledged $1 million to defend the ban, lawmakers said.

Other state proposals to ban abortion are before legislatures in Mississippi, Georgia, Indiana, Missouri, Ohio, Kentucky and Tennessee.

With files from The Associated Press