A grizzly bear roams through the Hayden Valley in Yellowstone National Park in Wyoming: Reuters

Three hunters were badly hurt in two separate grizzly bear attacks in Montana on the same day, state officials have said.

All three suffered “moderate to severe” injuries after being mauled by a lone bear in the Gravelley Mountains on Monday, according to the Montana Fish, Wildlife and Parks agency.

It is not known if the same grizzly was involved in both attacks, which happened less than 12 hours apart in the same area southwest of the city of Bozeman.

In the first attack, a bear charged at two men hunting in the mountain range at 7.30am on Monday, officials said.

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One of the pair was able to drive the grizzly away with bear spray while it was “on top of” his friend, according to a worker at an outdoors shop the men visited later that day.

The employee, Terri James, said the hunters thought they may have startled the bear as it was sleeping.

“They said he was up and in their face, just right there on them before they even had a chance to really even grab a gun or do anything, which I think is important for people to know,” she told NBC Montana.

She added: “The bear was on top of one of the men, and the other one was able to get to his bear spray. He sprayed the bear, and that’s what made the bear leave.”

The men sought medical treatment in the small town of Ennis. After leaving hospital they visited the shop to buy new clothes and seemed in high spirits, according to Ms James, who posted pictures of the bloodied hunters on Facebook.

The third man was attacked at 6.30pm while hunting with a friend, who was unharmed. They drove the bear away, and the injured hunter was treated in the village of Sheridan before being transferred to the city of Butte.

Montana Fish, Wildlife and Parks said both attacks remained under investigation. Wildlife authorities sometimes capture and kill bears that have become habituated to people, and also do so more rarely when an animal is believed to have intentionally sought out its victims.

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Officials urged people to be cautious and carry bear spray when grizzlies are active in spring, summer and autumn.

The grizzly population in and around Yellowstone National Park, which includes the Gravelly Mountains, has increased from an estimated 136 when they were granted federal protection in the mid-1970s to about 700 today.

Last year, a bear and its cub fatally mauled a hunting guide in northwestern Wyoming.

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