A search is intensifying through difficult, rugged terrain and cold temperatures for a missing 3-year-old boy in North Carolina, officials said.

"Our number one priority is to find Casey," Craven County Sheriff Chip Hughes told reporters Wednesday.

Casey Lynn Hathaway went missing from the backyard of his grandmother's home Tuesday afternoon, Hughes said, according to ABC New Bern affiliate WCTI.

Authorities are searching Casey Lynn Hathaway, a missing 3-year-old boy.(Craven County Sheriff's Office) Authorities are searching Casey Lynn Hathaway, a missing 3-year-old boy.

Casey was playing with two other children, and when those kids came inside without Casey, the grandmother and others spent 45 minutes searching before calling 911, authorities said, according to WCTI.

The U.S. Marines Corps has sent in people to help with the search, Hughes said Wednesday.

The family has been extremely cooperative, Hughes added.

The sheriff's office has also asked people to check their property, cars and storage sheds for Casey.

"No tip is insignificant," the sheriff said Wednesday.

"We're not giving up," added the county's emergency management coordinator, Stanley Kite.

Casey has blonde hair and brown eyes. He stands at 28 inches tall and weighs 25 pounds, the sheriff's office said.

Anyone with information is urged to call the Craven County Sheriff's Office at 252-633-0498 or the North Carolina State Bureau of Investigation at 919-662-4500.

ABC News' Ahmad Hemingway contributed to this report.