The Second Avenue Subway's 72nd Street cavern as of Jan. 21, 2016. View Full Caption MTA Capital Construction/Rehema Trimiew

UPPER EAST SIDE — The MTA needs another $10 million in back-up funding per month to finish the Second Avenue Subway by its December deadline, officials said.

Construction costs have already reached $39 million per month, but the MTA now needs to spend roughly $49 million each month to complete the project on time, the project's independent engineer, Kent Haggas, said during the MTA's transit and bus committee meeting on Monday.

Contractors said last month that they've only hit 70 percent of the milestones that the MTA set in March and that they have yet to complete escalator and elevator installations at the 72nd Street station.

In addition, there's a backlog of changes that came up during construction that the MTA still needs to make to move forward, according to the engineer. But he didn't get into details about what those changes entail.

"Given the amount of time left, we would like all of those targets to be met," Haggas said. "The contractors are under obligation to meet the accelerated schedule."

The extra funds will be taken from the MTA's existing contingency budget. The agency's full board already voted to use $66 million from that budget in February to keep the project on track, leaving $50 million left.

"Increasing the amount of money spent on construction shows a positive trend which means the contractors have committed to putting out the manpower to make their deadlines before December," Haggas said.