Charlotte Sena was camping with her family in Moreau Lake State Park, Saratoga County, when she disappeared on Saturday evening while on a bike ride by herself.
A major search has since been launched, with more than 100 people including police, forest rangers and civilians looking for the little girl.
Drones, bloodhounds, diving teams and an airboat were also being used in the search, officials said on Sunday.
Charlotte was last seen riding her bike around a loop trail in the secluded country park, which covers more than 6,000 acres.
The schoolgirl had been cycling with other children when she decided to ride one more lap by herself, Governor Kathy Hochul said at a press briefing on Sunday. “[She had wanted to] be that big girl, do it by herself”.
“That’s really when the nightmare begins,” she said.
Charlotte was last seen around 6.15pm. When she failed to return 15 minutes later, “her parents knew immediately something was up” and launched a search.
Her mother called 911 after Charlotte’s bicycle was found around 6.45pm on Saturday.
Officials issued an Amber Alert on Sunday morning, while state police Lt Colonel Richard Mazzone said it is “quite possible that an abduction had taken place”.
The alert describes Charlotte as a white girl with blonde hair and green eyes, about 4ft 6in tall.
The youngster, a resident of nearby Greenfield, was last seen wearing an orange tie-dye Pokemon shirt, dark blue trousers, black Crocs and a grey bike helmet.
Gov Hochul described Charlotte’s parents, David and Trisha, as “distraught”.
”As a mother and as a grandmother I cannot imagine the pain they’re going through,” she said.
She said Charlotte and her two sisters were raised just 15 minutes from the “beautiful, scenic” park, which lies around 35 miles north of state capital Albany.
“On a picture-perfect fall day here in upstate New York, Charlotte Sena’s family and friends gathered for a weekend of friendship and food and enjoying one of the last few days of good weather...so the kids could have a chance to be in nature, have a chance to be kids,” said Gov Hochul.
“They were here to make memories - the kind that last a lifetime. But instead the day turned into every parent’s nightmare.
“We are leaving no stone, no branch, no table, no cabin unturned, untouched, unexamined in our search to find Charlotte,” she added, describing Charlotte as a “joyful” child who was recently elected a class officer for student council.
“Everybody thought of her as just a really nice girl - the kind everyone wanted to be friends with, the kind that looked out for other children,” she said. “The pride of every parent.”
“Hearts are broken here today in New York.”