Officers have carried out extensive searches since Nathan Ilunga went missing in Withernsea on Friday, but they have been unable to trace him.
The teenager was last seen on South Promenade at around 3.20pm that day.
A police spokesman said: "He does have links to London and the Thames Valley area, so he may have attempted to travel south.
"However, we’re concerned for his safety, so if you believe you have seen him, please get in touch."
Nathan is described as 5ft 4in and slim. He was last seen wearing a black North Face baseball cap, a two-tone grey puffa jacket, olive green tracksuit bottoms and black Nike trainers.
Anyone with information is urged to call 101, quoting log 387 of March 23.