A man engulfed in flames was hospitalized with critical injuries Wednesday after leaping from the fourth-floor window of a burning Bronx apartment where another man was found dead, witnesses and FDNY officials said.

FDNY Fire Marshals have yet to determine what sparked the 7:45 a.m. fire at the Montgomery Ave. building near W. 174th St. in Morris Heights.

As the fire raged, witness Uneek Valentin, 46, watched from the window of her fifth-floor apartment as her neighbor made his death-defying jump to escape the flames.

“I heard him screaming. I thought he was on the roof, but then I saw him in his apartment window. He was on fire. He was screaming in agony. He broke the window, and then there was a plume of black smoke,” said Valentin, who lives with two of her four kids in a fifth-floor apartment in the building.

“He threw himself out the window.”

That victim, a 31-year-old man, was taken to Jacobi Hospital in critical but stable condition, cops said.

Officials could not immediately identify the fatal fire victim, who was found dead when firefighters breached the smoldering apartment. Multiple neighbors said the dead man was the critically injured victim’s uncle. The pair lived in the same apartment.

“(The man who jumped) is still alive, but his uncle was trapped in the fire,” said Valentin. “He was killed.”

Five more civilians and four firefighters were also injured in the fire, according to fire officials.

The fire remained under investigation Wednesday.

The blaze quickly grew to two alarms and spread to an adjacent apartment, two apartments above, and the cockloft between the top floor ceiling and the roof, FDNY officials said.

Wind may have contributed to how quickly the fire spread. In total, 140 firefighters and EMS personnel responded to the scene, officials said.

“I ran into my kid’s room. When I opened the door to their room, it was filled with black smoke,” said Valentin. “I rushed to get them out. I’ve been burned out of my apartment. Everything has been destroyed. I don’t know what I’m going to do.”

The Red Cross has helped 65 people left homeless by the fire find temporary housing, officials said.

“I ran into my kid’s room. When I opened the door to their room, it was filled with black smoke,” said Valentin. “I rushed to get them out. I’ve been burned out of my apartment. Everything has been destroyed. I don’t know what I’m going to do.”

The Red Cross has helped 65 people left homeless by the fire find temporary housing, officials said.