Mayor Bill de Blasio on Tuesday tried to reassure jittery commuters that he has their back on the brutal 15-month shutdown of the L train that starts in April.

Amid worries over prolonged repair work on a subway line that serves 225,000 riders each weekday, the mayor promised a “war room dynamic” to move people between Brooklyn and Manhattan.

“I’m here today to say, ‘We got you. We are on this,’ ” the mayor said at a press conference in East Williamsburg, which he reached via an L train ride from Union Square in Manhattan.

“This is going to cause some real dislocation,” he added. “It’s not going to be easy, it’s not going to be quick — it’s going to take 15 months — but I can say with assurance, people will be able to get around.”

The shutdown was prompted by subway tunnel damage from Superstorm Sandy in 2012.

The city is adding shuttle buses, ferry service and promoting bike-riding along the L train route — which crosses Manhattan at 14th Street.

City Council member Stephen Levin, who represents Greenpoint and parts of Williamsburg, said many riders will take the inconvenience in stride if their commutes don’t grow by more than 20 minutes each way.

“Any more than that and it’ll start to be an issue,” he said.