US President Donald Trump's administration have annulled an Obamacare provision that obliged employer health plans to pay for contraception.

The move extends an exemption, which was already given to religious institutions, to all commercial enterprises.

It expands "exemptions to protect moral convictions for certain entities and individuals whose health plans are subject to a mandate of contraceptive coverage" under Obamacare, a note published by the US Department of Health and Human Services said.

The move has drawn fire from reproductive rights advocates and praise from conservative Christian activists.

California's Democratic Attorney General pledged to fight to protect the mandate from circumvention.

It remains unclear how many women would lose contraception coverage and which companies would use the exemptions.

"The Trump administration just took direct aim at birth control coverage for 62 million women," Planned Parenthood Federation of America President Cecile Richards said in a statement.

"This is an unacceptable attack on basic healthcare that the vast majority of women rely on. With this rule in place, any employer could decide that their employees no longer have health insurance coverage for birth control," Ms Richards added.

Obamacare is the common name for the Affordable Care Act, health reforms that took effect under former President Barack Obama in 2010.  The act allowed millions of uninsured people to get health insurance.

Repealing the act was one of Mr Trump's most strident campaign promises.

He described Obamacare as a "total disaster," but his Republican Party has failed in efforts to repeal the health reforms.

Millions of American women who had the cost of contraception reimbursed could be affected by the decision, which conservative groups had been seeking since Obamacare began.

Challenges to Obamacare reached the US Supreme Court, which in 2014 ruled that family-owned private companies could choose not to provide contraceptive coverage to female employees on religious grounds.

In May, Mr Trump signed a decree on religious liberty ordering his administration to take account of objections of conscience on matters of contraception.