Two teenagers have life-threatening injuries after being mauled by a grizzly bear while on a survival skills course in the Alaskan wilderness.

The group of seven students, who were on day 24 of a 30-day backpacking course in the Talkeetna mountains, north of Anchorage, had been in single file for a river crossing when the animal attacked. The two teenagers at the front of the line, Joshua Berg, 17, from New City, and Samuel Gottsegen, also 17, from Denver, suffered the most serious injuries.

Witnesses said the animal struck out at Berg first, turned to another student, then turned back to Berg. The pair were being treated at Providence Alaska medical centre in Anchorage. Both were in a serious condition, said a hospital spokeswoman, Crystal Bailey.

The group was rescued early on Sunday after activating their emergency locator beacon. Staff from the Rescue Co-ordination Centre, operated by the Alaska Air National Guard, were called at around 9.30pm in response to the signal but a state trooper and helicopter pilot only found the students in a tent nearly six hours later.

Four of the teenagers were taken to hospital in Palmer city, about 40 miles northeast of Anchorage, where they received emergency treatment, but the rescue team decided the two most seriously injured would have to wait for medically trained crew.

Bruce Palmer, a spokesman for the National Outdoor Leadership School, which runs the course and organises excursions in Alaska and elsewhere, said: "Our basic goal is that when a student graduates from the course, they have the experience and background to be able to take other people out into the back country. We're training people to be outdoor leaders."

Among the skills learned on the course is the practice of calling out, to alert bears of human presence in order to give animals a chance to flee. "The students say they attempted that," Palmer said.