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

The new rules issued on Friday mark another step in the rolling back of the health care law often known as Obamacare.

The new policy is a long-expected revision to US federal rules requiring most companies to cover birth control as preventive care for women, at no additional cost.

Preventive services are supposed to be free for employees and their dependants under former president Barack Obama's Affordable Care Act.

Mr Trump's religious and moral exemption is expected to galvanise both his opponents and religious conservatives that back him, but it is likely to have a limited impact on America's largely secular workplaces.

Most women no longer pay for birth control, and advocates immediately announced plans to try to block the new rule in court.

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

Human resource managers recognise employers get an economic benefit from helping women space out their pregnancies, since female workers are central to most enterprises.

The administration estimated 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.

However, it is unclear how major religious-affiliated employers such as Catholic hospitals and universities will respond.

Since contraception became a covered preventive benefit, the share of women employees paying their own money for birth control pills has plunged to under 4%, from 21%, according to the Kaiser Family Foundation.

The Trump administration's revision broadens a religious exemption that previously applied to houses of worship, religiously affiliated nonprofit groups, and closely-held private companies.

Administration officials said the new policy defends religious freedom.

Privately owned for-profit companies, as well as publicly-traded for-profit companies will be able to seek an exemption.

Officials also said the administration is tightening oversight of how plans sold under the health law cover abortion.

With limited exceptions, abortions can only be paid for through a separate premium collected from those enrolled.

No public subsidies can be used, except in cases that involve rape, incest, or preserving the life of the mother.

Doctors' groups that were key to derailing Republican plans to repeal the health law outright expressed dismay over the administration's move on birth control.

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

"Instead of fulfilling its mission 'to enhance and protect the health and well-being of all Americans', leaders under the current administration are focused on turning back the clock on women's health," said the organisation's president, Dr Haywood Brown.

Women's groups said they would try to stop the changes.

"The rules give employers a licence to discriminate against women," said Fatima Goss Graves, president of the National Women's Law Centre.

"We will take immediate legal steps to block these unfair and discriminatory rules."

Administration officials said the new policy takes effect right away.