North Shore Medical Center is scaling back the expansion of its Salem campus, citing ongoing financial challenges and uncertainty about future demand.

The change would include reducing the number of new psychiatric patients the hospital can treat, even while demand for such services remains strong. The decision surprised state regulators, who had approved the original plans a few months ago.

The hospital, owned by Partners HealthCare, seeks to add 30 fewer psychiatric beds and 16 fewer medical/surgical beds to its main campus in Salem. As previously disclosed, the hospital is also set to close its smaller campus in neighboring Lynn by 2019, a move opposed by many in the community.

North Shore Medical Center plans to spend about $207 million to renovate and expand in Salem over the next three years, about $33 million less than previously planned.

The expansion plans have been in the works for years. But in a staff memo, the chief executives of Partners and North Shore Medical Center said they now prefer a “phased approach’’ to the project, to ensure “that we can remain good stewards of limited resources — bringing on beds as demand and resources warrant.’’

North Shore Medical Center has struggled financially for years. It lost $36 million in the fiscal year that ended Sept. 30, 2015, and $22 million the year before.

State health officials, who signed off on the hospital’s plan in July, were puzzled at the sudden change. They noted that the project was approved in part because it was pitched as a way to meet the local demand for psychiatric care.

“The proposed reduction comes at a time when the Commonwealth has been experiencing significant demand for psychiatric services,’’ Department of Public Health spokesman Tom Lyons said in an e-mail.

The revised plan needs approval from state health officials.

“DPH is surprised by how quickly the project is coming back to the department,’’ Lyons added.

North Shore Medical Center had been planning to add 56 psychiatric beds. Now it is planning to add about 26 but would build room to add more beds eventually, depending on demand.

There are about 2,650 psychiatric beds in Massachusetts, excluding state-run mental health facilities, according to the Massachusetts Association of Behavioral Health Systems. That number is growing, thanks to projects under way across the state, but advocates and state health officials say that demand for mental health services continues to outstrip the supply.

Laurie Martinelli, executive director of the Massachusetts chapter of the National Alliance on Mental Illness, said the pared-back plan at North Shore Medical Center is problematic. “Seeing a reduction in the number of planned psychiatric beds at NSMC is disturbing,’’ she said by e-mail, “because there are never enough psych beds.’’

Changes to the hospital’s expansion plan were reported earlier by the Salem News.

Priyanka Dayal McCluskey can be reached at priyanka.mccluskey@globe.com. Follow her on Twitter @priyanka_dayal.