Firefighters battling the deadly inferno in The Bronx ran into a roadblock when a fire hydrant outside the building froze in the bone-chilling temperatures, a high-ranking police source said Friday.

It took firefighters hours to bring the raging fire under control after the first 911 call was made at 6:51 p.m.

“The fire department had a problem because the fire hydrant in front of the building was frozen,” the source said.

The fast-moving blaze at 2363 Prospect Ave. in the Belmont neighborhood of the borough killed a total of 12 adults and children.

Nearly 200 smoke-eaters responded to the five-alarm blaze in below-freezing temps.