The annual military drills, which both South Korea and the US claim are defensive in nature, kicked off on Monday after the North's fourth nuclear test and rocket launch earlier this year.
China's foreign ministry spokesman Hong Lei told reporters that Beijing takes note of North Korea's harsh verbal threats against the military drills.
"China is also very concerned about the military drills," Hong said, Yonhap News Agency reported.
"China is firmly opposed to any action that causes trouble" on the Korean Peninsula, Hong said. "We will not accept any trouble-making behavior on our doorstep."
North Korea threatened pre-emptive and "indiscriminate" nuclear strikes to cope with the South Korea-US drills.
Asked about the North's nuclear threats, Hong urged the "relevant parties" to exercise restraint.
North Korea's harsh rhetoric against the annual drills between South Korea and the US are not uncommon, but it came days after the UN Security Council adopted tougher sanctions to punish the North for its nuclear test and rocket launch.
China's cooperation was of key importance to ensure the adoption of the new UN sanctions on North Korea.
China is a major importer of North Korean coal and iron ore, but Hong said the new UN resolution "has a definite target that is to curb North Korea's nuclear and missile programs without impacting the well-being and humanitarian needs of the North Korean people."
"All parties should fully and earnestly implement the resolution without exacerbating or neglecting any aspects," Hong said.