Chinese and Russian military forces are engaged in joint exercises in north-western China as ties grow between the two autocratic states amid uncertainty over instability in Afghanistan.
The exercises involving ground troops and air forces are due to continue through until Friday in the Ningxia Hui Autonomous region.
The region borders on Xinjiang, where China has detained more than one million Uyghurs and members of other Muslim minorities in what it calls a campaign against terrorism and extremism.
Xinjiang shares a narrow frontier with Afghanistan, and Beijing is concerned about violence spilling over its border if the Taliban take control in the country following the pullout of US and NATO troops.
While not part of a formal alliance, Russia and China have aligned their military and foreign policies largely in opposition to those of the US and its allies.
The official Xinhua News Agency said the exercises began on Monday and were presided over by Li Zuocheng, a member of the ruling Communist Party's Central Military Commission.
The exercise aims to "deepen the joint anti-terrorism operations between the Chinese and Russian militaries and demonstrate the firm determination and strength of the two countries to jointly safeguard international and regional security and stability", Xinhua said, citing Chinese and Russian officials.
"It reflects the new height of the China-Russia comprehensive strategic partnership of co-ordination for a new era and of the strategic mutual trust, pragmatic exchanges and co-ordination between the two countries," Xinhua said.
Russia has backed China in its claim to virtually the entire South China Sea, over which Beijing clashed with common rival the US at a high-level UN Security Council meeting on maritime security.
China, Taiwan and Asean members Brunei, Malaysia, the Philippines and Vietnam have overlapping claims in the disputed waters and have been locked in increasingly tense territorial standoffs for decades.
China built seven disputed reefs into missile-protected island bases in recent years, ratcheting up tensions with rival claimants, along with the US and its allies.
US secretary of state Antony Blinken responded to China's increasingly assertive claims to the strategic waterway by warning that any conflict there or in any ocean "would have serious global consequences for security and for commerce".
The area has seen "dangerous encounters between vessels at sea and provocative actions to advance unlawful maritime claims" that seek to "intimidate and bully other states lawfully accessing their maritime resources", Mr Blinken said.
China's deputy ambassador, Dai Bing, responded by accusing the US of becoming "the biggest threat to peace and stability in the South China Sea" and calling its "hype" in the Security Council "entirely politically motivated".
China has refused to recognise an international arbitration ruling in 2016 that invalidated most of its claims in the South China Sea.
Cover image: Xinhua/Alamy.