BEIJING—China and Russia will hold joint naval exercises in the South China Sea in September, the Chinese Defense Ministry said, amid heightened regional tension following an international tribunal’s rejection of Beijing’s maritime claims there.

China’s defense-ministry spokesman, Senior Col. Yang Yujun, gave few details at a monthly news conference on Thursday but said the drills were “routine” and not directed at any other countries.

However, they are the first joint exercises in the South China Sea between China and Russia, which have been strengthening defense ties in recent years in part due to a shared interest in countering pressure over their military activities from the U.S. and its allies.

The joint drills will be the first scheduled by any countries in the South China Sea since the tribunal at the Permanent Court of Arbitration in The Hague ruled overwhelmingly against China on July 12 in a case brought by the Philippines over Beijing’s maritime claims.

“This is a routine exercise between the two armed forces, aimed at strengthening the developing China-Russia strategic cooperative partnership,” Col. Yang said. “The exercise is not directed against third parties.” Russian media reported the Chinese announcement but there was no immediate confirmation from the Russian defense ministry.