The US State Department expressed concern Thursday about a series of retaliatory measures China has been taking against South Korean businesses over Seoul's decision to host the US THAAD missile defense system, calling the reaction "unreasonable and inappropriate."

"We are concerned and closely monitoring reports that China has taken actions against South Korean private sector entities for the US.-ROK (South Korea) decision to deploy THAAD in the ROK," a State Department spokesperson told Yonhap News Agency on background.

"As THAAD is a prudent and limited self-defense measure designed to respond to a clear, reckless and unlawful North Korean military threat, criticism or pressure on the ROK to abandon its self-defense would be unreasonable and inappropriate," the official said.

The spokesperson also said that the US stands by its "ironclad commitment" to the defense of allies, and will "continue to develop a comprehensive set of alliance capabilities to counter the growing North Korean ballistic missile threat."

It is the first time that the US has officially denounced China's reprisal measures against THAAD's deployment. That shows the administration of President Donald Trump has been taking a tougher stance on Beijing than the previous Barack Obama administration.

 South Korea and the US decided last year to deploy a Terminal High Altitude Area Defense (THAAD) battery in the South to defend better against the North as the communist nation stepped up development of ballistic missiles of all ranges.

China has strongly called for scrapping the decision, seeing THAAD, especially its powerful "X-band" radar, as a threat to its nuclear deterrent and other security interests, despite repeated assurances from Washington that the system is designed only to defend against the North. 

Beijing has taken a series of economic measures in apparent retaliation for the decision, such as import bans on a number of South Korean products, tax and other inspections against Korean businesses and tightening of regulations on Korean cultural products and tourism. (Yonhap)