Humana's decision to exit the Obamacare exchange market in 2018 is setting off alarm bells for health insurers and regulators, even as the Trump administration issues new rules to try to stabilize the exchanges.

"I think you will see a lot more withdrawals," said Aetna CEO Mark Bertolini, at a Wall Street Journal health care event, one day after Aetna and Humana agreed mutually to end their merger agreement.

"There isn't enough money in the ACA as it's structured, even with its fees and taxes, to support the population that needs to be served," he explained, adding that without government funding, similar to Medicare, to help insurers with the costs of covering patients with high medical bills, the exchange market will not work. "It is in a death spiral."

Separately, the health department released Wednesday a set of rules for 2018 that, among other things, tightened enrollment rules in an effort to prevent people from signing up for coverage and then dropping it after incurring high medical bills.

"I think we should be encouraged by the signal that the administration wants to stabilize the market," said Andy Slavitt, the former director of the Centers for Medicare and Medicaid under the Obama administration, but he thinks insurers need much more clarity on bigger issues.

"Right now, the greatest uncertainty is whether or not the individual mandate, which is the principal driver to help keep costs lower, is going to be enforced," he explained. "Within the next month, insurers have to start submitting rates and deciding where they're going to participate. It's very hard to do that if you don't know the rules of the road. It's very hard to do that if you don't know how your competitors are interpreting the rules of the road."