WASHINGTON (Reuters) - The Trump administration on Thursday issued a final rule that will shorten the Obamacare enrollment period and give insurers more of what they say they need in the individual insurance market, likely making it harder for some consumers to purchase insurance, healthcare experts said.

An emergency sign points to the entrance to Scripps Memorial Hospital in La Jolla, California, U.S. March 23, 2017. REUTERS/Mike Blake

It could also raise out-of-pocket medical expenses, the experts said, because it gives insurers more flexibility in determining the value of their coverage.

The rule, which takes effect later this year, comes as President Donald Trump and Republicans in Congress have renewed efforts to repeal and replace the Affordable Care Act, commonly known as Obamacare, after an effort to pass a bill in the U.S. House of Representatives failed last month.

Issued by a division of the U.S. Department of Health and Human Services and first proposed in February, the rule aims to aid insurers, who have lost hundreds of millions of dollars in the individual insurance markets set up by Obamacare. Several major insurers, including Humana Inc and Aetna Inc, have announced plans to exit some state exchanges in 2018.

Insurers welcomed the rule but said there is still too much uncertainty in the market. On Wednesday, Trump told the Wall Street Journal that he may withhold Obamacare payments to insurers that amount to about $7 billion a year to force Democrats back to the negotiating table.

Marilyn Tavenner, president and chief executive of America’s Health Insurance Plans, said in a statement that funding for the payments must continue uninterrupted. Otherwise, she said premiums will rise 20 percent across the market and more insurers would drop out of the exchanges.

The changes under Thursday’s final rule include a shortened open enrollment period for Obamacare plans. They also make it harder for people to enroll outside that period, which is allowed under certain circumstances such as a pregnancy or a move.

The rule could also allow insurers to collect unpaid premium payments and make it tougher for people to move in and out of insurance plans, according to healthcare industry experts.

Insurers say “gaming the system” has created an unprofitable mix of healthy and sick customers.

The rule also gives states broader authority by removing the federal government’s role in overseeing doctor and hospital networks included in insurance plans. Republicans have said any healthcare reform or overhaul must give states more flexibility.

The Affordable Care Act enabled 20 million Americans to gain insurance, mostly through the individual insurance markets set up by the law or through an expansion of Medicaid, the government health insurance program for the poor and disabled.