GUANGZHOU, Aug. 21 -- Customs in southern China recently seized 600 tonnes of smuggled electronic cigarette oil, with a total value of 300 million yuan (about 44 million U.S. dollars).

Over 320 police raided four groups who were suspected of smuggling the oil from the United States, according to Zhou Bin, head of Gongbei Customs Office, which administers Zhuhai and Zhongshan cities in Guangdong Province.

The four companies were based in Shenzhen and Xiamen and supplied the majority of the e-cigarette oil in the Chinese market, according to Zhou.

In recent years, sales of electronic cigarettes have grown by more than 300 percent annually, but supervision of the industry is still weak, Zhou said.

Most of the oil sold in China is imported, he said.

Twenty people have been placed under criminal detention following the raid, and further investigation is underway.