Russian police have launched a full-scale search for an American postgraduate student who has vanished in a mountainous area of Siberia.

Colin Madsen, 25, left a guesthouse in Arshan, in the Buddhist region of Buryatia, in the early hours of Sunday morning, and has not been seen since.

Police are using a helicopter to search for the missing US citizen from Jefferson City, Missouri, who is currently studying at Irkutsk State Linguistic University.

He left the guesthouse between 2am and 5am, without a coat, despite subzero temperatures, according to a police statement, reported The Siberian Times.

Colin Madsen, 25, left a guesthouse in Arshan, in the Buddhist region of Buryatia, in the early hours of Sunday morning, and has not been seen since. He is described as 5'6" tall, of slim build, with blue eyes and brown hair

Police are using a helicopter to search for the missing US citizen from Jefferson City, Missouri, who is currently studying at Irkutsk State Linguistic University. He left the guesthouse without a coat, police say

Friend Vlad Rasputin, from Irkutsk, wrote on his social media account: 'I'm going to Arshan right now, and will help in the search.

'At the moment no-one can clearly explain what happened.

'We have called the US Embassy in Moscow and they told us that they are taking measures.'

Another friend, Elena Fuks, said: 'Today they should be searching for him with a helicopter.'

The student's mother, Cale Madsen, is flying to Russia to help the search for her missing son, she said.

He is believed to have been staying at Arshan, a tourist resort which also has a prominent Buddhist temple, with Russian friends.

'Police officers are searching for a 25-year-old man, who went out of a guest house in Arshan village on 27 March, between 2 am and 5 am and has not yet returned,' said a statement from Buryatian police.

He is believed to have been staying at Arshan (pictured), a tourist resort which also has a prominent Buddhist temple, with Russian friends. He was dressed in sand-coloured trousers, a T-shirt, and wore grey hiking boots

Temperatures in the region are around 9C during the day, but dip below freezing at night

He is described as being 5'6" tall, of slim build, with blue eyes and brown hair.

He was dressed in sand-coloured trousers, a T-shirt, and wore grey hiking boots.

'If you have information on the whereabouts of the missing man please notify us,' said a statement.

Temperatures in the region are around 9C during the day, but dip below freezing at night.

Arshan, at an altitude of 2,950 feet, is famous for its hot springs and waterfalls.