US President Donald Trump is allowing more employers to opt out of providing no-cost birth control to women by claiming religious or moral objections.

Key points: The changes allow companies to opt out of providing birth control at no cost to employees

The changes allow companies to opt out of providing birth control at no cost to employees The ACLU has flagged a court challenge

The ACLU has flagged a court challenge Experts say it's another step to weaken 'Obamacare'

The Trump administration's new rules take another step in rolling back Obama-era health care laws.

Employers with religious or moral qualms will also be able to cover some birth control methods, and not others.

Experts have warned that could interfere with efforts to promote modern long-acting implantable contraceptives, such as IUDs, which are more expensive.

The new policy was a long-anticipated revision to Affordable Care Act (ACA) laws which require most companies to provide birth control as preventive care at no additional cost.

That Obama-era requirement applies to all FDA-approved methods, including the morning-after pill, which some religious conservatives call an abortion drug, though scientists say it has no effect on women who are already pregnant.

As a result of the ACA, most women in the US no longer pay for contraceptives.

Several advocacy groups immediately announced plans to try to block the Trump administration rule.

"We are preparing to see the government in court," said Brigitte Amiri, a senior attorney for the American Civil Liberties Union.

Catholic bishops called the administration's move a "return to common sense."

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Mr Trump's religious and moral exemption is expected to galvanise both his opponents and religious conservatives who back him.

But it seems unlikely to have a major impact on America's largely secular workplaces.

"I can't imagine that many employers are going to be willing to certify that they have a moral objection to standard birth control methods," said Dan Mendelson, president of the consulting firm Avalere Health.

But Mr Mendelson said he worried the new rule would set a precedent for weakening ACA requirements that basic benefits be covered.

"If you look at it as a public health issue, it is a step in the wrong direction, and it weakens the protections of the ACA," he said.

Tens of thousands of women could be affected by Mr Trump's policy, but the vast majority of companies have no qualms about offering birth control benefits through their health plans.

The administration estimated that some 200 employers who have already voiced objections to the Obama-era policy would qualify for the expanded opt-out, and that 120,000 women would be affected.

The United States Conference of Catholic Bishops welcomed the administration's decision.

"Such an exemption is no innovation, but instead a return to common sense, long-standing federal practice, and peaceful coexistence between church and state," Cardinal Daniel N DiNardo, the group's president, said in a joint statement with Archbishop William E Lori of Baltimore, head of its religious liberty committee.

Doctors' groups that were instrumental in derailing Republican plans to repeal the health law expressed their dismay.

The American Congress of Obstetricians and Gynaecologists said the new policy could reverse progress in lowering the nation's rate of unintended pregnancies.

"HHS leaders under the current administration are focused on turning back the clock on women's health," said the organisation's president Dr Haywood Brown.

The new rules take effect right away.

AP/ABC