Mayor Eric Adams added another notch to the city’s fiscal belt this week, ordering most agencies to leave thousands of positions vacant as the city looks to close a $2.9 billion budget gap in the next fiscal year — exacerbated by the city’s $1 billion migrant crisis and deals cut with municipal unions.

A Monday letter from Budget Director Jacques Jiha to agency heads said the Mayor’s Office of Management and Budget would be reducing full-time positions that were vacant at the end of last month by 50%.

The cuts mean that about 4,700 vacancies would be left unstaffed, but the directive will not affect teachers or uniformed members of the police and fire department.

The missive also directed agencies to self-fund new initiatives and programs and “submit detailed proposals” outlining the funding by Dec. 5.

The news came after Adams ordered a 3% budget slash in September intended to save $2.5 billion over the next two fiscal years, on top of additional sweeping cuts the incoming mayor made in January.

Jiha cited pending settlements with the city labor unions and the New York’s estimated $1 billion bill for sheltering migrants as the basis for the new cuts, in addition to broader economic woes.

“Until the federal and state governments step in, we are shouldering the entire cost of providing mandated assistance to asylum seekers,” he wrote.

“Furthermore, the local and national economies are weakening, financial market performance is down, and we face rising health-care costs, high energy prices, and elevated inflation.”

City spending surged about 3% to $104 billion in fiscal year 2023, which began this summer. The city’s nearly $3 billion dollar deficit could swell to $6 billion in fiscal year 2026, according to budget projections.

The increase prompted a dire warning from New York Comptroller Tom DiNapoli, who said the bloated budged “could create spending pressure” in future years, as he called for “balancing” the need to deliver municipal services with filling the city’s coffers.

“Without the hardworking individuals who keep this city running, critical and essential services for our children and most vulnerable residents could be impacted,” DiNapoli warned earlier this month.