A South Korean teenager who vanished in Turkey crossed the border into Syria voluntarily, Seoul police said Wednesday, amid mounting suspicions that he may have joined the Islamic State militant group.

The 18-year-old, identified only by his surname Kim, disappeared on Jan. 10 during a trip to the southern Turkish town of Kilis and was last seen leaving a hotel.

A Turkish newspaper reported earlier that he had exchanged emails with the extremist Muslim militant group IS and probably crossed into Syria to join them.

Investigators at the Seoul Metropolitan Police Agency probing Kim's disappearance said they reached the tentative conclusion that the teenager had sneaked into Syria by himself.  

"It has been confirmed so far that Kim's disappearance was not related to an abduction," Jeong Jae-il, a senior SMPA official probing the case, said.

Despite the possibility that Kim joined IS, he said, there is no clear evidence so far supporting that possibility.

"Many materials suggest that he was greatly interested in IS. However, police cannot confirm whether he had actually joined them," said Jeong.

It would mark the first known occurrence of a South Korean national to join any radical Muslim militant group if Kim were found to have joined the group voluntarily.  

After searching Kim's personal computer, the investigator said Kim accessed IS-linked websites and exchanged dialogue with a Turkish friend he met online known as "Hassan."

Kim's parents allowed him to travel to Turkey, along with a family friend, after he pleaded to make the trip, the official said, adding that they were unaware that Kim had wanted to meet Hassan in person.

Also, police said Kim posted on his Facebook account on Jan. 7 that he wanted to leave South Korea to search for a new life.

"I want to leave this country and family. I just want a new life," Kim said.

The SMPA also said that Kim made one phone call on Jan. 9 and another on Jan. 10 before taking a taxi with a Syrian license plate, together with an unidentified man after meeting him in front of the hotel in Kilis.

Kim and the unknown man then got off near a refugee camp in Besiriye, about 18 kilometers southeast from Kilis, on that day, according to the official. (Yonhap)