Russia is to hold joint naval exercises with China in the South China Sea in September as tensions escalate over the fiercely contested islands.

China's Defence Ministry insisted the "routine" drills were aimed at strengthening their co-operation and were not aimed at any other country.

But they come at a time of heightened tensions in the region after a UN-backed tribunal ruled this month that China did not have historic rights to the waters.

China rejected the ruling and refused to participate in the case.

"This is a routine exercise between the two armed forces, aimed at strengthening the developing China-Russia strategic cooperative partnership," China's defense ministry spokesman Yang Yujun told a news conference.

"The exercise is not directed against third parties."