The trafficking of women and children into China continues to rise despite efforts to combat it, a senior police officer has said.

Chen Shiqu, who heads the government agency tackling trafficking, warned that traffickers still pose a serious threat and called for greater international cooperation to end the scourge.

He declined to give a specific number of victims who fall prey to traffickers annually, only saying that 1,500 cross-border cases involving about 2,000 kidnapped women and infants had been uncovered since 2009.

Most victims were smuggled to the Chinese mainland from Vietnam, Myanmar and Laos, he said.

"Great demand from buyers, as well as the traditional preference (among Chinese families) for boys, are the main culprits fueling trafficking," Chen, director of the Ministry of Public Security's (MPS) anti-human trafficking office, told China Daily.

In recent years, Asian nations have agreed on several joint projects to tackle the cross-border smuggling of women and children.

In 2004, China, Cambodia, Laos, Myanmar, Thailand and Vietnam signed the Mekong River Sub-regional Cooperation Anti-trafficking Memo and set up an annual meeting of senior officials.

Beijing has also established eight border offices with neighboring countries, Chen said. However, he acknowledged that differences in legal systems, language and collecting evidence are obstacles in the fight against trafficking.

"Countries should rise above legal differences, and enhance intelligence and information communication," he said, adding that his office will strengthen cooperation in extraditing suspects and transferring case materials with counterparts in neighboring countries.

In describing the plight of victims, Chen explained that some rescued women had been sold as brides in Yunnan and Guangdong provinces, whiles others were forced into prostitution in border areas of Yunnan and the Guangxi Zhuang autonomous region.

Most kidnapped infants were trafficked to Guangdong and Guangxi for illegal adoption, he said, adding that boys sold for as much as 40,000 yuan ($6,000), while girls went for 30,000 yuan.

In some areas of Guangdong and Guangxi, couples with no sons bought and illegally adopted abducted children as "boys carry on the family line" and "raising sons can guarantee one's old age".

Most trafficking suspects from Vietnam and Myanmar are immigrants living illegally in China. Many of them are women who live in Guangdong and Guangxi and are consequently much more familiar with local custom and market demand.

"The female suspects tend to be more discreet," Chen said. "They colluded with local 'black agencies' and had a relatively stable market there."

Abducted infants are susceptible to infections and often fall ill with colds and fevers and sometimes suffer more serious ailments.

Last month Chinese police uncovered one major cross-border child trafficking ring. Eight Vietnamese infants kidnapped for sale were rescued and 39 suspects were arrested.

Most of the eight abducted infants, aged from 10 days to seven months, had been sedated with sleeping pills to stop them crying, and some were suffering serious health problems, including coughs, fever and hydrocephalus, swelling of the brain, when rescued.

Traffickers had abducted the children from Vietnam and transported them on bamboo rafts across the Beilun River and then over the border on a long journey to Guangdong.

They rode bicycles through the fields to bypass border checkpoints, and arrived at Dongxing and Fangchenggang in Guangxi. They then boarded long-distance buses to Nanning, the capital of Guangxi.

From Nanning, they took the children to Shanwei and Jieyang in Guangdong to sell them, boys for 40,000 yuan, and girls for 20,000 to 30,000 yuan.

All the rescued infants from Vietnam underwent medical treatment in China. They have been placed in welfare institutions by the civil affairs departments and their birth parents will be located and verified through DNA tests, according to Chen.

"We have officially informed the Vietnamese police and asked for judicial assistance to find their birth parents as soon as possible," he said.

Li Guifang, deputy director of the criminal defense committee at the All China Lawyers Association, called for harsher penalties for convicted traffickers.

"Chinese judicial authorities should increase the punishment for the buyers, and completely cut off the trafficking chain to make the buyers lose both the children and the money," he said.