China's top uniformed officer has warned that the military "will not be soft-handed" if anyone attempts to "separate" Taiwan from China.

The Central Military Commission's vice chair, General Zhang Youxia, spoke on Monday at the opening ceremony of the Beijing Xiangshan Forum.

Delegates from more than 90 countries and organizations are taking part, including Russian defense minister Sergei Shoigu and US Defense Department officials in charge of China affairs.

The forum, organized by groups related to the Chinese military, is being held without a defense minister after Li Shangfu was dismissed last week.

General Zhang said China will "deepen strategic cooperation with Russia" and is "willing to develop military ties with the US."

But he warned that countries should not deliberately provoke other nations on major and sensitive issues.

The US and China recently traded accusations over each other's military activities around the South China Sea.

Washington accused a Chinese fighter jet of conducting an unsafe intercept of a US bomber. Meanwhile, Beijing alleged that a US destroyer crossed in front of a Chinese destroyer near a group of islands it claims.

Attention is on whether participants from the two sides will have contact during the forum.