A 15-year-old boy has been rescued after spending days sitting atop a boat that had capsized in the frigid waters of Alaska's Bering Sea.

The boy, identified by family members as Derek Parker Aghnaanga, was the lone survivor from the skiff, having lost his older brother and cousin in the incident.

The trio had departed from St Lawrence Island on Friday 4 September and were due back the following Sunday afternoon, the US Coast Guard said. But when the 18-foot fishing vessel failed to return, they launched a search effort.

A Coast Guard aircrew located the boy sitting on top of the overturned boat on Monday morning, and a nearby good Samaritan vessel pulled him to safety.

That vessel was also able to recover the bodies of his two relatives.

"Our hearts go out to the families, friends, and communities affected by the tragic loss of these two mariners," said Rear Adm. Bob Little, the commander of the Coast Guard's Arctic District.

The boy had signs of hypothermia when he was rescued, the USCG said.

The captain of the fishing boat that pulled Aghnaanga to safety told a local Anchorage news outlet that the teen had clung to the skiff through darkness and water temperatures around 50F (10C).

"That's one tough kid," the captain, Adam White, told KTUU-TV.

"Through the teeth chattering, he had mentioned that his brother was still under the skiff," White added. "And then he said that, in the previous night, before our rescue, that his cousin had slipped away."

Their fishing boat capsized on Saturday, leaving Aghnaanga without food or water for two days at sea, the boy's mother wrote on Facebook.

Vina Kulowiyi, the mother of Aghnaanga and his older brother, said her sons were doing what they loved: fishing to provide for members of their community who don't always have access to meat at their local store in Savoonga.

Savoonga - a village of around 800 residents, located on St Lawrence Island off Alaska's coast - is primarily inhabited by Siberian Yupik peoples. English and St Lawrence Island Yupik are the island's main languages.

"My boys were born and raised to be providers. A tradition that is passed on," Kulowiyi wrote on Facebook. "We hunt and provide for families and share the community."

Kulowiyi said her surviving son is recovering at home.

"My survivor," she wrote. "His story is amazing. He was never afraid the whole time."

He sat "praying the whole time", she added.