SHIJIAZHUANG, May 6 -- Police in a northern Chinese province have rescued and returned six Vietnamese women to Vietnam after breaking a human trafficking case involving 34 Vietnamese women.

The public security department of Hebei Province said on Wednesday that the 34 women were sold to several villages in Hebei, beginning in 2012.

In May 2014, police rescued a Vietnamese woman who was sold in Hebei's Xinle City and forced to marry a Chinese man, according to the department. Following her clues, police arrested a man surnamed Lv and his wife, who confessed to making 400,000 yuan (64,500 U.S. dollars) from trafficking 34 Vietnamese women.

The department did not reveal any more information about the rest of the victims, only saying the rescue operation is ongoing.

Police say a rise in drug trading, smuggling and human trafficking along the China-Vietnam border has accompanied a strengthening of economic ties between the two countries in recent years.

Most of the trafficked women have ended up as prostitutes or purchased wives of rural Chinese men.

Last month, Chinese police returned to Vietnam a 26-year-old Vietnamese woman who was sold into marriage in China at the age of about 17.