China is sending civilian militias to help secure the border it shares with North Korea, state media said, in the wake of two reported killings of Chinese citizens by North Koreans that could strain ties between Pyongyang and its sole major ally.

The China Defence News said on Wednesday it had established a civilian-military defence system in the Danubian prefecture of Jilting province. Danubian shares a border of about 500 km (310 miles) with North Korea.

"China and North Korea are both keeping guard on the border," the newspaper said. "The situation is more complicated and relying on just one party would make it difficult to achieve effective control."

The government has also "guided the establishment of militia patrols" to guard border villages. Every 10 neighboring households would have their own border security group and there would be 24-hour video surveillance, the newspaper said.

Last week, China said it had lodged a protest with North Korea after media reported that a North Korean army deserter had killed four people during a robbery in the Chinese border city of Helong late last month.

State media has raised questions about the China-North Korea relationship, saying that the Chinese government "should not be too accommodating".

China is North Korea's most important diplomatic and economic ally, although three nuclear tests, several rounds of sabre-rattling and violence on the China-North Korea border have tested Beijing's support.

The 520 km-long Tumen River that divides China and North Korea is a popular route used by defectors fleeing the secretive North.