The 25-year-old student -- identified as Colin Madsen on his social media pages -- disappeared early Sunday after arriving in the mainly Buddhist region of Buryatia in eastern Siberia for a hiking trip, the regional Investigative Committee, which probes major incidents, said in a statement.
The US student was staying overnight with a group of friends in the village of Arshan at the foot of the Sayan Mountains, located west of Lake Baikal.
"While the others were sleeping, he left the house and disappeared without trace," the Investigative Committee said.
His friends, including another US student, spent the day searching for him before contacting police, investigators said.
The missing student has been studying in the eastern Siberian city of Irkutsk at the local branch of Moscow State Linguistic University since 2013 and speaks fluent Russian. He had previously visited Arshan four times.
Police issued a missing person report on Monday, saying the student disappeared between 2 and 5 am while wearing only a T-shirt, trousers and climbing boots.
Friends wrote on his page on Russian social networking site VK that his mother is expected to arrive in the region this week.
Natalia Tugutkhonova, who works for a tour company, wrote on Facebook on Tuesday that "a friend and just a very good person is missing.
She said Madsen was not answering his cellphone and that "his friends and relatives are very concerned for him."
Another friend, Vlad Rasputin, wrote on VK on Wednesday that he had arrived in Arshan to help the search effort. The weather "is very cold, it's snowing and there is strong wind," he wrote.
The US embassy told AFP: "We were aware of these reports. However, for privacy reasons, we cannot provide any further details."
"The Department of State takes its obligation to assist US citizens abroad seriously," it added.
