Media playback is unsupported on your device Media caption ABC News' Chuck Sivertsen: "It was the third grizzly bear attack in the Yellowstone region in the past year"

A female grizzly bear has killed a man out on a hike with his wife in the US after the couple apparently surprised the bear and its cubs, officials say.

The attack took place on the Wapiti Lake Trail, in an isolated area near the middle of Yellowstone National Park, officials said.

The woman survived despite being chased by the bear and lifted into the air by her rucksack, she told park officials.

It was Yellowstone's first fatal bear mauling since 1986.

The couple had seen the bear earlier on Wednesday but continued hiking, said park spokesman Al Nash. The second time they saw it, the bear was coming towards them.

"The husband reportedly told his wife to run and she did so. And that's when the bear attacked and killed the man," Mr Nash said.

The woman told officials she did not witness the fatal attack on her husband, and had dropped to the ground when the bear chased after her.

It had lifted her off the ground by her rucksack before dropping her and making off, Mr Nash quoted her as saying, leaving her bruised but not seriously hurt.

Another group of hikers nearby heard the woman's cry for help, and called 911 to summon the rangers.

Bear pepper spray

Campsites in the area have been closed, warning signs posted and wardens have been working to clear hikers from the area.

But Mr Nash said it was unlikely that park rangers would take any action against the bear.

"This was probably a defensive attack on the part of the bear, that the bear felt threatened and attacked the person," he said. "If that's the case... we don't have any plans to track or dart the bear".

He described the incident as "extremely rare" and urged hikers to stick to designated trails, walk in numbers, make lots of noise and carry a bear pepper spray.

"This is the first time we have had a human killed by a bear in Yellowstone in 25 years," he said. "Simply seeing a bear in the distance does not necessarily require you to run away. You just need to be careful and mindful."

More details were expected later on Thursday after the victim's family has been notified.

While Yellowstone park itself has not experienced a fatal bear attack in 25 years, this is the third bear attack in the region in just over a year.

A year ago, a grizzly killed one man and wounded two others in a night-time attack on a campground just outside the park.

A month earlier, there was another attack near the park gate, reports said.

Yellowstone National Park and the surrounding area in the north-western United States are home to several hundred grizzly bears, which can weigh up to 600lb (270kg) and are a big tourist attraction.