A southern Chinese city is on the alert for piranhas after two people were attacked in a river, and is offering a 1,000 yuan ($160) reward for every fish caught, dead or alive, state media reported on Thursday.

The government of Guangxi region's Liuzhou is asking people to hunt the alien South American species, which badly bit two people earlier in the week who were paddling in the Liujiang River, the China Daily said.

"Fishing with nets is not allowed in the section of the river that flows through the city, but we have made an exemption. Five fishing boats with experienced fishermen have been deployed on the river since Monday," Liuzhou official Wei Yongwen told the newspaper.

"In addition, more than 40 other fishermen from the local fishing association have joined us as well. They all use small pieces of pork as bait."

Other people have taken up position along the river's banks with rods, it added.

"It's horrible to know that the river has such fish. I will not swim there anymore," resident Liu Junjie was quoted as saying. "I'll pray they catch them soon."

However, their days may be numbered anyway, as piranhas die when the water temperature drops below 15 degrees Celsius (59 degrees Fahrenheit), as it will do in Guangxi over the winter, the China Daily added.

Chinese media has said the piranhas may have been released by people who had bought them as ornamental fish, and that authorities are now stepping up patrols of markets to ensure no more are sold.