A U.S. police officer didn't think twice when the life of a small boy with autism was on the line.

Topeka Police Department Officer Aaron Bulmer was in a neighbourhood near the city’s Central Park Sunday on an unrelated call, looking for suspects in a fight.

That's when he spotted a four-year-old boy wandering alone in the park.

Bulmer says he circled around in his squad car and then noticed the boy heading toward a pond.

The officer turned on his body camera and dashed out of his car toward the boy, but couldn’t reach him fast enough.

Video shows the boy bobbing and struggling in the freezing cold water, and Bulmer immediately jumps in to pull him to safety.

"I need some help, call 911 for me,” he can be heard saying on the video.

"He almost drowned. I got him out."

In a press conference a few days later, Bulmer talked about his heroic efforts, saying he still gets emotional when he thinks about it.

"I get choked up. I go back and watch the video a few times,” he says, pausing. “It could have been my kid."

"I jumped into action just like anyone would have."

Local media say the boy's parents were looking for him at the time he fell in and say he's OK after the near-drowning thanks to Bulmer's quick actions.