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China said it was investigating the killings of three villagers in a border town where North Korean guards have been accused of crossing over to commit thefts and slayings

The killings were under investigation and border security troops will "assist police in handling border security related issues and ensure stability along the China-North Korean border," defence ministry spokesman Geng Yansheng said on Thursday.

The Helong city government said a 55-year-old man, his 26-year-old daughter and another man aged 67 were killed on Saturday. It said in a statement on Thursday that police from the provincial level down were investigating.

The killings took place in Longcheng township along the North Korean border. The location is not far from where at least seven villagers were reported killed last year by border guards who had crossed the Tumen River, which separates the countries, in search of money and food, the AP news agency reported.

China is North Korea's chief source of trade and aid, but the North's desperate economic situation is testing the traditionally close relationship. Underpaid and underfed border guards have reportedly been terrorising Chinese villagers in recent years, especially in winter when the frozen river makes crossing over easy.

More than 1 million North Koreans are believed to have died in a famine in the 1990s and the country remains heavily dependent on food aid, although the political elite and military receive preferential rations.

Border attacks are believed to have grown over the past two years after a crackdown on crossings by ordinary North Koreans deprived guards of income from bribes. Chinese fishermen have also been held for ransom after being seized by North Korean boats in the Yellow Sea dividing the countries.

The incidents underscore a substantial cooling of ties since Kim Jong-un took over as supreme leader of North Korea in 2012.

China has been repeatedly rebuffed in its attempts to convince the North to rejoin multinational talks on its nuclear programmes, exposing it to criticism for supporting the government in Pyongyang. Beijing also was shocked by the 2013 execution of Kim's uncle Jang Song-thaek, who had been a key proponent of North Korea-China relations.

The 1,400km border between the sides runs through mountainous and often-remote terrain from the Yellow Sea to the Sea of Japan.

Source: AP