BEIJING/SEOUL (Reuters) - Chinese Foreign Minister Wang Yi has criticized South Korea's decision to deploy an advanced U.S. anti-missile defense system to counter threats from North Korea, saying it harmed the foundation of their trust.

The announcement by South Korea and the United States this month that they would deploy a Terminal High Altitude Area Defence (THAAD) unit has already drawn protests from China that it would destabilize regional security.

The decision is the latest move to squeeze the increasingly isolated North Korea, but China worries the system's radar will be able to track its military capabilities. Russia also opposes the deployment.

"The recent move by the South Korean side has harmed the foundation of mutual trust between the two countries," Wang was quoted by South Korean media as telling South Korean Foreign Minister Yun Byung-se.

China's foreign ministry, in a later statement, cited Wang as saying that South Korea should think twice about the deployment and value the good momentum of ties between Beijing and Seoul.

"THAAD is most certainly not a simple technical issue, but an out-and-out strategic one," Wang said late on Sunday on the sidelines of a conference of foreign ministers from the 10-member Association of South East Asian Nations in Vientiane.

Yun told Wang the deployment was aimed at protecting South Korea's security and that it would not damage China's security interests, South Korea's Yonhap news agency said.

In a meeting with North Korean Foreign Minister Ri Yong Ho, who is also in Laos, Wang said China and North Korea were traditional friends, and China was committed to the Korean peninsula's denuclearization and to resolving problems through talks, the ministry added.

At a separate meeting in Beijing, Fan Changlong, one of the vice chairmen of the Central Military Commission that controls China's military, told U.S. National Security Adviser Susan Rice that the THAAD deployment would only worsen tension on the Korean peninsula.

"The United States must stop this kind of mistaken action," China's Defence Ministry cited Fan as saying.

South Korea and the United States have said THAAD would only be used in defense against North Korean ballistic missiles.

North Korea has launched a series of missiles in recent months, the latest last week when it fired three ballistic missiles that it said was a simulated test of preemptive strikes against South Korean ports and airfields used by the U.S. military.

The missiles flew 500-600 km (300-360 miles) into the sea off its east coast and could have hit anywhere in South Korea if the North intended, the South's military said.

North Korea came under the latest round of U.N. Security Council sanctions in March after its fourth nuclear test in January and the launch of a long-range rocket the following month.

(Reporting by Jack Kim, additional reporting by Ben Blanchard in Beijing; Editing by Paul Tait, Robert Birsel)