SEOUL, Jan. 21 -- A South Korean teenager is estimated to have joined the Islamic State as Seoul police said Wednesday the 18-year-old has crossed the Turkish border into Syria.

The Seoul Metropolitan Policy Agency (SMPA) announced its investigation results into the teenager, identified by his surname Kim, saying he crossed the border into Syria voluntarily.

Kim left South Korea for Turkey on Jan. 8 on his first trip overseas with a South Korean guide, and moved to the southern Turkish town of Kilis, about 5 km away from the Syrian border, a day later.

On Jan. 10, the teenager left a hotel at around 8 a.m. and got on a van with an unidentified Arab man. They were dropped off at a camp for Syrian refugees near Besiriye, a Turkish town some 18 km east of Kilis.

Since then, Kim has disappeared. A Turkish newspaper reported that a South Korean national joined the IS group, bolstering mounting suspicion that the teenager could have entered the Islamic militant group.

Seoul police said it hasn't been verified whether Kim joined the IS group. If confirmed, he would become the first known South Korean jihadist.

The police, however, said Kim has searched the keywords of IS, Turkey, Syria and Islam on the Internet more than 500 times since January 2014. His personal computer saved four photo files describing IS members with an IS flag on their hands. He deleted 47 photo files depicting IS members with rifles on their hands and Islamic women.

The teenager, who dropped out of his middle school, is known to have stuck to a trip to Turkey.

Kim's parents were unaware that he was to visit Turkey to meet a Turkish friend known as "Hassan" who Kim met online.

Kim is believed to have contacted IS-linked people through social network services such as Facebook, Twitter and Surespot. In March 2014, he posted a message saying "I want to enter IS" on his Facebook account.

On Jan. 9, a day after he left for Turkey, Kim said on his Facebook account that he "wants to live a new life away from" his family and nation.