The city will spend an additional $690 million for “new needs” identified in just the past two months — including $250 million to battle the ongoing homelessness crisis, officials said Tuesday.

The details were unveiled during Mayor de Blasio’s presentation at City Hall of an $84.7 billion preliminary budget for Fiscal Year 2018 — up 3.1 percent from $82.1 billion in 2017.

The boost in spending spurred financial watchdogs and several City Council members to express concerns about the continued budget expansion — particularly in the face of uncertainty about funding from the federal government.

“[The budget] continues an expansion of the scale of municipal government to a record size,” said Maria Doulis, an analyst at the Citizens Budget Commission. “A significant portion of this growth is consistent with the implementation of the administration’s policy agenda . . .”, she added.

But she said another portion “derives from challenges managing serious problems, notably an inability to stem the growth in homelessness and in the reliance on overtime among city agencies.”

Records show that 60,469 people are now being housed in city shelters.

And in one year, the budget for the Department of Homeless Services soared from $1.1 billion to $1.4 billion — which includes the $250 million added Tuesday.

Other new expenses include $52 million for overtime costs at the Department of Correction and $21 million in OT at the Fire Department, budget officials said.They said overtime costs at those agencies would be capped moving forward.

The city is also earmarking $20 million per year for new safety measures at homeless shelters, which for the first time are being overseen by the NYPD.

But de Blasio noted that the city’s budget cushion was at or near a high point — with roughly $5.2 billion in reserves earmarked in FY 2018.

He pointed to $1.1 billion in new savings identified for FY 2017 and 2018 — though much of it was generated from cost re-estimates rather than cost-cutting — and said he’d ask agencies to identify $500 million in additional savings by June 30.

“This is a budget that represents the values of this administration,” de Blasio said. “We think it gets at the concerns that everyday New Yorkers raise to us all the time.”

Hizzoner also said the budget made no assumptions about the federal government’s funding commitments to the city, given the lack of clarity from President Trump and congressional leaders on the issue.

The city has time to make adjustments.

The mayor’s “executive budget” is due at the end of April, and that, too, is subject to amendment.