An increasing number of employees at school bus companies operating between the cities of Hong Kong to Shenzhen are smuggling mobile phones over the border, according to Luohu Customs.

The school bus employees are also forcing students, who study in Hong Kong but live in Shenzhen, to smuggle mobile phones in for them.

On the afternoon of Sept 9, customs officials found two uniformed women, surnamed Chen and Wang, leading a group of students from Hong Kong to Shenzhen without proper documentation.

The customs officials detected something suspicious when up to four iPhones were found in the women's underwear as they and the group of students passed a gate at the border between the two cities.

It is the second case in a month that border-crossing shuttle bus employees have tried to smuggle in mobile phones, according to the customs office in Luohu, a district in Shenzhen.

On Aug 15, customs caught eight workers who intended to smuggle in a total of 64 mobile phones, worth more than 300,000 yuan ($49,000), in students' school bags.

The smugglers might face criminal prosecutions, according to the press department of Luohu Customs. But the children who helped the employees smuggle in the phones, many of whom are under the age of 14, will not face any punishment.

The customs said it has urged shuttle-bus operators to improve their management of workers.

Customs will also now keep strict records of shuttle-bus operators and provide training sessions to the companies' staff. It will increase examinations of students and their shuttle-bus escorts when they cross the border every weekday.

The latest figures show that about 16,000 children in Shenzhen cross the border to study in Hong Kong every day, many of whom use the services of shuttle buses.

Chen Wenli contributed to this story.