Graphics: GT

China has "lodged representations" to North Korea after a North Korean army deserter killed four Chinese citizens during a robbery in the border city of Helong, Yanbian Korean Autonomous Prefecture in Jilin Province.

Chinese foreign ministry spokeswoman Hua Chunying said on Monday that China's public security ministry is investigating the case.

The North Korean army deserter crossed the border into China on December 27, 2014. He allegedly stole money and food from a village before killing four residents, according to reports by South Korea's Yonhap News Agency and Dong-A Ilbo newspaper.

Sources said that the soldier has been detained just north of the Tumen River, a boundary river between China and North Korea.

China's Ministry of Public Security declined to comment on the case, saying "the information is sensitive."

According to the Dong-A Ilbo newspaper, the victims were two elderly ethnic Korean couples. Nearly 30 percent of the population in Yanbian is of Korean ethnicity.

Lü Chao, an expert at the Liaoning Academy of Social Sciences, told the Global Times that armed robberies by North Koreans have happened many times in the area and have victimized both guards on patrol and local residents.

"Local villagers have taken measures to protect themselves," Lü said, adding that the number of such incidents has been declining, and that killings such as the one in Nanping are quite rare compared to robberies over the past 10 years.

According to The Beijing News, the township of Nanping in Helong, where the killing took place, has witnessed some 20 villagers killed by armed North Koreans. The report did not specify a time frame.

Over 70 percent of the population in Nanping is Korean-ethnic.

Jin Qiangyi, director of the Asia Studies Center at Yanbian University, said hunger and poverty are the main factors driving North Koreans across the border.

"These people are not real defectors, who will pass into China quietly and keep a low profile. The hungry North Koreans searching for food will eventually go back to their country. Most Chinese citizens won't report them," Jin said.

Zhao Lixin, another Korean studies expert based in Yanbian, said that these people, as well as defectors, are usually sent back to North Korea after being captured. But the alleged criminal should not be handed back to the North, he said.

In December 2013, a North Korean man killed an elderly Chinese couple in the Chinese border city of Yanji and stole 20,000 yuan ($3,210). He was caught by Chinese authorities after fleeing to Beijing.

Cases like this should be viewed as a criminal incident rather than a political case, Zhao said. "The Chinese government will protect its citizens' safety. This will not influence the relations between China and North Korea."

A businessman surnamed Du living the city of Tumen told the Global Times that he and his friends have seen many hungry young North Korean men coming to villages near Tumen. "The numbers increase in winter because they can simply walk across the frozen Tumen River."

Du said local residents, mostly ethnically Korean Chinese, give food to North Koreans out of sympathy.

A villager, Wang Yuying, who runs a timber business in Changbai county at the foot of Changbai Mountain, told the Global Times that robberies occurred frequently as North Koreans began to sneak into the county in the 1990s.

"Since then, local residents became cautious and were warned not to go the mountain alone, fearing that they could bump into armed North Korean soldiers or be kidnapped," Wang said.

Security measures in border areas in Jilin Province have been tightened after the incident, Dong-A Ilbo said, quoting sources in Jilin.

But a police officer from Changbai county denied the report, arguing that security is tight and incidents are few. "If North Koreans are attempting to run away from their country, their police will detect them first," the officer said on condition of anonymity.