NANJING -- A court in eastern China has sentenced 23 people to prison for smuggling refined oil, authorities said Friday.

The gang members were fined and sentenced to jail terms up to 15 years, while their smuggling vessels, two oil tankers, were confiscated, according to Nanjing Customs, citing a statement from Yangzhou City Intermediate People's Court in Jiangsu Province.

In November last year, Yangzhou Customs staff found an oil tanker coded "Haiguanshan No.66" was illegally transporting refined oil products in ports along the Yangzhou section of the Yangtze River. Further investigation showed another oil tanker was also involved.

Yangzhou Customs found that the smugglers had asked overseas ships to dock in international waters, and sent the oil in Chinese ships. Financial transactions were conducted via underground banks.

The gang had smuggled more than 35,000 tonnes of oil products since 2013, customs staff said.