BEIJING (Reuters) - China and Russia will hold "routine" naval exercises in the South China Sea in September, China's Defence Ministry said on Thursday, adding that the drills were aimed at strengthening their cooperation and were not aimed at any other country.

The exercises come at a time of heightened tension in the contested waters after an arbitration court in The Hague ruled this month that China did not have historic rights to the South China Sea and criticized its environmental destruction there.

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 regular monthly news conference.

"The exercise is not directed against third parties."

China and Russia are veto-wielding members of the U.N. Security Council, and have held similar views on many major issues such as the crisis in Syria, putting them at odds with the United States and Western Europe.

Last year, they held joint military drills in the Sea of Japan and the Mediterranean.

White House spokesman Josh Earnest played down the significance of the exercises even though he conceded that the South China Sea was "a sensitive diplomatic topic right now".

"I don't know what exercises they are planning, but in the same way the United States and China have a military-to-military relationship, I'm not surprised that Russia and China are seeking to build upon their military-to-military relationship as well," he told a regular briefing.

China has recently taken part in U.S.-led multinational naval drills in the Pacific and a U.S. defense official said he did not expect the China-Russia exercises to affect U.S. military activity or behavior in the South China Sea.

“We're not concerned about the safety of U.S. vessels in the region as long as interactions with the Chinese remain safe and professional, which has been the case in most cases,” the official said.

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China claims most of the South China Sea, through which more than $5 trillion of trade moves annually. Brunei, Malaysia, the Philippines, Taiwan and Vietnam have rival claims.

China has repeatedly blamed the United States for stoking tension in the region through its military patrols, and of taking sides in the dispute.

The United States has sought to assert its right to freedom of navigation in the South China Sea with its patrols and denies taking sides in the territorial disputes.

Russia has been a strong backer of China's stance on the arbitration case, which was brought by the Philippines.

Yang said China and Russia were comprehensive strategic partners and had already held many exercises this year.

"These drills deepen mutual trust and expand cooperation, raise the ability to jointly deal with security threats, and benefit the maintenance of regional and global peace and stability," he said.

(Reporting by Ben Blanchard in Beijing; Additional reporting by David Brunnstrom and Idrees Ali in Washington; Writing by John Ruwitch and Brenda Goh; Editing by Robert Birsel and James Dalgleish)