NEW YORK (Reuters) - Enrollment on the Obamacare health insurance exchanges in 2018 and later years is expected to be less than previously forecast, dampened by the Trump administration’s decision to pull back enrollment efforts, the U.S. Congressional Budget Office said on Thursday.

The administration has cut back on advertising spending, halved the enrollment period to six weeks, and reduced outreach.

In addition, insurance premiums will rise sharply in 2018, the CBO said, which will also deter enrollment.

The non partisan agency now estimates that the average monthly enrollment on the exchanges will be about 11 million people, down about 4 million from last year’s forecast. People can sign up for or drop out of coverage throughout the year as their employment status or access to other government health plans changes.

Republicans have tried to replace Democratic former President Barack Obama’s 2010 Affordable Care Act, often called Obamacare, but have been unable to muster the needed votes to pass a new law.

The administration of President Donald Trump has created further uncertainty by refusing to commit to paying the program subsidies that make the individual insurance affordable, which has caused insurers to raise rates for next year to cover these potential costs.

Insurers must decide this month whether to stay in this market and many, such as Anthem Inc, Aetna Inc and Humana Inc have pared back or completely exited the market for next year.

This is the first full update since March 2016 from the CBO on anticipated government spending on the subsidized individual health insurance program. It said about 10 million people have insurance through the program next year.

In March 2016, the CBO expected about 18 million people to be in these plans purchased through the marketplace in 2026. It now expects about 12 or 13 million people to purchase their insurance that way.

The agency has given enrollment outlooks under several Republican healthcare bills that did not pass that would also have seen enrollment decline.

The agency said that it expects the individual health insurance market, even as it faces sharp premium increases and fewer insurers next year, to be stable in most areas of the United States. It expects premiums to increase in this market on average about 5 percent per year between 2017 and 2027, and to rise about 15 percent in 2018 versus 2017.