(Reuters) - Mississippi’s governor signed into law on Monday the most restrictive abortion measure in the United States, which was immediately challenged in court by abortion rights advocates who say it is unconstitutional.

Republican Governor Phil Bryant said he was proud to sign the bill banning abortion after 15 weeks of gestation with some exceptions, according to a statement from spokesman Knox Graham.

“I am committed to making Mississippi the safest place in America for an unborn child, and this bill will help us achieve that goal,” Bryant said.

The law takes effect immediately. Previous Mississippi law banned abortion at 20 weeks after conception, similar to limits in 17 other states.

Abortion rights advocates have said the measure targets the state’s only abortion provider, the Jackson Women’s Health Organization, which provides abortions for up to 16 weeks after conception.

The Center for Reproductive Rights said it had filed suit in U.S. District Court on behalf of the clinic to block the Mississippi law. It said the law violated Supreme Court precedent that a state may not ban abortion before the fetus can survive outside the womb.

“Politicians are not above the rule of law, and we are confident this dangerous bill will be struck down like every similar attempt before it,” Nancy Northup, the Center for Reproductive Rights’ president and chief executive, said in a statement.

The U.S. Supreme Court legalized abortion in its 1973 Roe v. Wade ruling. The country’s highest court has since banned the prohibition of abortion before fetal viability, usually seen at about 20 weeks of gestation.

A similar measure banning abortion 15 weeks after conception has been introduced in the Louisiana legislature. The Mississippi law includes an exception in the case of severe fetal abnormality or a medical emergency.

Abortion rights groups say anti-abortion organizations could use the legal case to test the limits of abortion all the way to the Supreme Court.

In 2016, the high court refused to uphold an Arkansas law that banned abortion after 12 weeks’ gestation as well as a North Dakota six-week law.

The Guttmacher Institute, which opposes abortion limits, has said about 926,200 abortions were performed in the United States in 2014, down 12 percent from 2011.

The Supreme Court is scheduled on Tuesday to hear arguments in a California free-speech case involving private facilities that counsel pregnant women against abortion.