HONG KONG (BLOOMBERG) - Hong Kong travellers to China are being asked to unlock their smartphones so Chinese agents can examine chat messages and social media, the latest move by authorities to prevent the financial hub's months-long protests from spreading.

One fund manager said he was told to unlock his phone this week at the Shenzhen border, after which officials inspected his Whatsapp and WeChat apps, as well as his photos. The agents asked how the fund manager had obtained pictures of the protests and why he had stored them, and took down information from group discussions about the demonstrations, said the person, who declined to be named.

The phone checks add to signs that Beijing is stepping up efforts to control the flow of information on the protests amid fears it could inspire similar unrest in China or embolden pro-independence forces in Taiwan. Domestic media coverage has been tightly controlled, and many cities are keeping a closer eye on citizens travelling to the former British colony.

The South China Morning Post earlier reported that 10 people said their phones had been checked while they were crossing into mainland China from Hong Kong.

Phone calls to China's Hong Kong and Macau Affairs Office were not answered. The National Immigration Administration and the Shenzhen-based agency handling border inspections do not provide contact information on their websites.

Officials at all Chinese ports of entry were told to proactively check the phones of suspicious-looking individuals entering from Hong Kong and delete pictures related to the protests, a person familiar with the directive said.

Bankers who travel frequently between Hong Kong and mainland China are swapping their personal phones and laptops for new devices or ones that have been wiped clean, according to three financial professionals in the city, who asked not to be named because of the security risk.

One of the bankers said she left her personal mobile phone containing images of the protests at home and took an old iPhone on a recent trip to Shenzhen. She also refrained from packing any outfits in black, the colour worn by the protesters, while her colleague deleted Whatsapp from their phone before crossing the border.

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Multiple Chinese cities this month began barring travel agents and couriers from helping individuals apply for the permits that Chinese citizens need to travel to Hong Kong. People now have to appear in person so their identities can be verified.

There are also indications China may be restricting travel from Hong Kong to the mainland. A group of Hong Kong students was told in July their application to enter the country as a tour group was denied because of the protests, a person familiar matter said.