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Police in the US searching for a missing Chinese exchange student, initially feared kidnapped, have said he could in fact be camping.

Kai Zhuang, 17, was last seen on Wednesday at his host family's home in the town of Riverdale in Utah.

Police originally said it is possible he is being held against his will.

Officers were alerted to concerns about Kai's welfare on Thursday by the school he was attending, which had been contacted by his parents in China.

Kai's parents in China claimed to school officials they had been sent a ransom demand and a picture of their son indicating he was abducted, according to an earlier statement from police.

But in a later press release, Riverdale Police Department said they have reason to believe Kai could be camping in a tent in the Wasatch Front area of Utah, according 2KUTV - an affiliate of the ´óÏó´«Ã½'s US partner CBS News.

Undated CCTV images released by the force on Saturday showed him walking through a shop. He reportedly bought camping supplies several times recently.

Police say they are worried Kai could be in danger because of the cold weather.

Officers also said he had interactions with police in nearby Provo on 20 December and had camping gear with him at the time.

In a press conference on Thursday, Riverdale police chief Casey Warren said there was no evidence Kai had been forcibly taken from his host home.

A missing persons alert was issued for the teenager.