(Reuters) - China said on Monday it had lodged a protest with Pyongyang after media reports said a North Korean army deserter killed four Chinese nationals during a robbery in the Chinese border city of Helong late last month.

Ministry spokeswoman Hua Chunying told a regular briefing in Beijing that China "would handle the matter in accordance with the law."

South Korea's Yonhap News Agency reported the deserter was detained by Chinese authorities just north of the Tumen River that divides China and North Korea. The 521-km long river in the far north is a popular breakout route used by defectors fleeing the secretive state.

The incident involving the armed North Korean occurred on Dec. 28, the report quoted unidentified sources as saying.

Dong-A Ilbo, a South Korean newspaper, said that North Korea had realigned its border troops after the incident.

South Korea's unification ministry said they could not confirm the reports.

Since he took power after the death of his father in 2011, North Korea's leader Kim Jong Un has strengthened border security to prevent defections, according to defectors and activists.