A rescue helicopter is lifting 20 stranded Northern Arizona University students and staff out of an area near the Grand Canyon's north rim.

Coconino County Sheriff's officials say the 16 students, two instructors and two aides were being flown out three at a time Tuesday afternoon to an area near Jacobs Lake.

The 20 were on a six-day backpacking trip for an outdoor leadership certification course.

Authorities say the group attempted to leave Monday afternoon, but its vans got stuck in the snow.

Group members were able to free the vehicles, but the snow-covered dirt roads were impassable and they called authorities.

They were instructed to drive as far east as possible and take shelter in the vehicles for the night.