Three senior high school students from Hubei province have gone missing after traveling to a border city in Yunnan province, police confirmed on Wednesday.

The three arrived in Xishuangbanna Dai autonomous prefecture, Yunnan province, on June 25, and lost contact with their families and friends two days later. The last known location sent from their mobile phones indicated they may have crossed the border, according to a statement by Huangzhou district public security bureau in Huanggang city, Hubei.

Authorities launched an investigation the same day the students were reported missing. A task force from Huangzhou district has been sent to Yunnan and is currently working with other departments to search for the teenagers.

Police have urged students and parents to remain vigilant during the summer holiday, warning against so-called zero threshold, high reward part-time job offers, which may be fraudulent or dangerous.