Mayor Bill de Blasio said "it's unacceptable" and "troubling" that an FDNY ambulance took nearly 21 minutes to respond to a fatal blaze on Easter that killed two 4-year-old siblings and injured three more people in Far Rockaway, Queens.

An city probe into what took so long and why is underway and should be done "in the next few days," the mayor told reporters Tuesday near City Hall.

"I do know -- we all know -- that something went wrong. We've got to know why it went wrong. We've got to know how to fix it going forward. It's not acceptable," the mayor said.

Sunday's fire, caused by a careless child playing with a lighter, killed Jai'Launi Tinglin and Aniya Tinglin, at the home at 10-31 Bay 30th St., according to the NYPD. The first call came in Saturday at 11:51 p.m., but the ambulance didn't arrive until about 12:12 a.m., official said.

De Blasio said: "We don't know yet what it would have meant if there had been a quicker response. We do know it's unacceptable that there wasn't a quicker response and we do know we have to address the issue, but we're going to need to complete the investigation."

FDNY spokesman Firefighter Michael Parrella said Tuesday night that the department's inquiry was ongoing.

Earlier this week, FDNY Commissioner Sal Cassano said: "We're listening to tapes, we're looking at timelines and at the end of the investigation we'll see what actions need to be taken."

Ambulances aren't generally dispatched until firefighters confirm they're needed, a fire spokesman said.