By Yereth Rosen

ANCHORAGE, Alaska (Reuters) - A 9-year-old boy who fell into a glacial crevasse in Alaska while riding a snowmobile is presumed dead and experts are working to recover his body, the Alaska State Troopers said on Sunday.

Sjohn Brown of Fairbanks, Alaska, fell into a 150-foot-deep (46-metre-deep) glacial hole on Saturday while he and family members were attending an annual snowmobile-racing event, the troopers said.

Rescue climbers descended to the bottom of the crevasse and found the snowmobile buried in the snow, as well as the boy's helmet and goggles, but not the child, the troopers said.

The boy is believed to be beneath the snowmobile and the snow, but a climber who reached the area was not able to locate a body using a probe, troopers spokeswoman Megan Peters said.

"The deep hole requires skilled climbers. We made an attempt but the hole has filled with a significant amount of snow," Peters said in an email.

Sjohn was with his father, who watched his son ride around a small mound and fail to reappear, Peters said. "The father rode to the location and found he had fallen through this hole," she said.

It is believed the boy fell into a glacial moulin, a circular shaft created by spring water flowing onto the glacial ice, Peters said.

The troopers were waiting on Sunday for approval for U.S. Army experts to attempt to extract the boy's body, she said.

"The skill needed to rappel down the hole is significant. We must have trained personnel to attempt a recovery," Peters said.

Sjohn and his family were attending the Arctic Man Classic, an event in which snowmobile-skier teams race each other in the mountains near Paxson, a community about 140 miles southeast of Fairbanks. The spring event usually attracts thousands of camping spectators.

(Reporting by Yereth Rosen; Editing by Edith Honan and Eric Beech)