"We recently completed the deployment of the first battery there. It achieved initial operating capability. That's not the full operational seamless capability that we want but it has an initial ability to be able to defeat North Korean missiles," he said.

Davis said China's opposition to the deployment is "perplexing."

"This is a system that is 100 percent defensive in nature. It does not have an offensive capability. It is meant to defend against North Korean missiles. That that should be considered a destabilizing act by others is curious and incomprehensible in my view," he said.

On Sunday, the North successfully test-fired a new intermediate-range ballistic missile. The test was seen as a demonstration of big progress the North has made in its pursuit of development of an continental ballistic missile capable of reaching the US.

The missile, which was launched at a high angle, reached the maximum altitude of more than 2,111.5 kilometers before landing in the East Sea 787 km away. Experts said the missile could have flown about 4,500 km if it had been fired at a normal angle. The distance to Hawaii, headquarters for US military operations in the Pacific, is 7,400 km.

"This weapons program that they have is unlawful and it presents a clear, grave threat to US national security. North Korea openly states that its ballistic missiles are intended to deliver nuclear weapons to strike cities in the US, the ROK, and Japan," Davis said.

"As we've spoken about with regards to China, North Korea is a liability for China, not an asset and China will partner with us to demonstrate to North Korea that it stands alone in its pursuit of illegal weapons of mass destruction," he said.

The official declined to discuss further specifics of the North's missile launch.

"We are still assessing this launch from over the weekend," he said. "I can't give you classified assessments of it but it's obviously a program that we continue to watch." (Yonhap)