SHANGHAI/HONG KONG (Reuters) - China has banned the bulk shipment to Hong Kong of black clothing and other gear used by pro-democracy protesters, staff at Chinese courier firms said, amid four months of often violent unrest in the city.

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Customer services staff at some of China’s major couriers, including STO Express, ZTO Express and YTO Express, told Reuters that the curbs were put in place around August.

One staff member from STO said that single pieces of black clothing could be sent to Chinese-ruled Hong Kong, but shipments of more than five pieces would be stopped. They were also not allowed to ship masks, bulk orders of umbrellas or sticks.

“Any items that can be used by mobs,” he said.

A Hong Kong-based customer service employee at S.F. Holding’s S.F. Express said it had not been informed of the new measures.

China’s courier network is made up of thousands of firms of which ZTO, YTO, STO, Yunda Holding and S.F. Express are among the biggest players. ZTO and YTO declined to comment, while STO said that it abided by China’s laws and regulations.

SF Express, Yunda Holding and China’s General Administration of Customs, which manages the country’s imports and exports of goods, did not immediately respond to requests for comment.

Black clothing and now-banned masks have been regularly worn by protesters in Hong Kong who have thrown petrol bombs at police, set street fires, trashed metro stations and public buildings and stormed the legislature. Police have responded with tear gas, rubber bullets, water canon and several live rounds.

The protesters are angry at what they see as Beijing’s tightening grip on Hong Kong, which was guaranteed 50 years of freedoms not extended to the mainland under a “one country, two systems” formula when Britain returned the city to China in 1997.

Beijing rejects the charge and accuses Western countries, especially the United States and Britain, of stirring up trouble. The unrest poses the biggest popular challenge to Chinese President Xi Jinping who has warned that any attempt to divide China will be crushed.

The shipping restrictions appear to be having little impact on the ground in Hong Kong, however, sales representatives at retailers such as Uniqlo, H&M and Giordano Ladies told Reuters.

Most said they had not heard of a ban on black clothing, and all said they had not received any corporate directives related to the ban, but also that they had not noticed an increase in the sale of black clothing.

One H&M manager, dressed in black, said she was surprised to hear of such restrictions. “We can sell black clothing,” she said. “We can also wear it.”