BEIJING: China "means what it says" when it says it will consider countermeasures against the planned US deployment of an advanced anti-missile system in South Korea, the defence ministry said on Thursday.

China, North Korea's neighbour and lone major ally, has repeatedly expressed anger at the United States and South Korea for their decision to deploy the US Terminal High Altitude Area Defence (THAAD) system in the South to counter missile and nuclear threats from North Korea.

The South Korean defence ministry said it would announce a new location for the system on Friday, after opposition from residents for the initial site choice.

Chinese Defence Ministry spokesman Yang Yujun said China's opposition to THAAD was clear.

"We will pay close attention to relevant developments, and consider taking necessary actions to protect national strategic security and the regional strategic balance," Yang told a monthly news briefing.

"What needs to be stressed is that Chinese people mean what they say," he added, without elaborating.

Beijing worries the system's radar will be able to track its own military capabilities, and that the deployment will do nothing to lower temperatures on the Korean peninsula.

Russian has expressed similar opposition.

North Korea, which has threatened a "physical response" against the THAAD decision, has conducted a series of military technology tests this year, including a fifth nuclear test this month, in defiance of UN Security Council sanctions that were tightened in March.