The deployment of the United States Terminal High Altitude Area Defense (THAAD) missile defense system in the Republic of Korea (ROK) is illegal and cannot protect the country, South Korean protesters said on Monday, adding that the move would intensify the arms race on the Korean Peninsula.

South Korean residents and peace activists gathered outside the presidential Blue House and the Defense Ministry's headquarters in Seoul on Monday to protest against further deployment of THAAD, after President Moon Jae-in ordered his aides to consult with the US about the deployment of four more mobile THAAD launchers. His order followed the DPRK's second launch of an intercontinental ballistic missile (ICBM) within a month.

Two THAAD launchers have already been deployed in Seongju county, about 300 km southeast of Seoul.

Among the protesters, a university student said the THAAD deployment would only heighten tensions on the Korean Peninsula as it irritates neighboring countries.

While the US and South Korea conducted joint military exercises near the peninsula and deployed the powerful anti-missile system in recent months, the DPRK has fired 18 missiles in 12 tests since February, including the latest ICMB potentially capable of threatening the American heartland.

"The currently deployed THAAD is illegal. The further deployment will never be accepted. We will stop THAAD on the road by mobilizing all available tools," said Lee Seok-ju, chief of Soseong-ri village in Seongju.

Washington claims that the THAAD deployment is aimed at protecting Seoul from Pyongyang's threat, but the move is strongly condemned by China and Russia.

On Saturday, Chinese Foreign Ministry spokesman Geng Shuang reiterated China's opposition, saying that the system's deployment seriously undermines the regional strategic balance and security interests of regional countries including China.

Another female peace activist called the THAAD deployment the "most serious wrongdoing in security and diplomatic affairs" by former president Park Geun-hye's administration. "How could President Moon, who was elected through the candlelit revolution, push for the THAAD deployment?" She asked.

She said the anti-missile system would not be able to defend the ROK from the DPRK's missile threats because the two sides are too close geographically and THAAD targets missiles flying at a high altitude.

Most of DPRK's missiles targeting South Korea fly at an altitude of less than 40 kilometers, but THAAD is designed to intercept missiles at an altitude of 40-150 kilometers, the Xinhua report explained, stressing that THAAD cannot protect the densely populated Seoul and its suburban areas.

Previous reports by South Korean media also suggested that THAAD could fail in case of an emergency.

The DPRK is developing a new-type of rocket artillery that can penetrate THAAD and hit Seoul, a report by South Korean newspaper The Dong-A Ilbo said in May, citing information provided by South Korean military personnel.

In addition, the DPRK has about 5,500 rockets which cannot be intercepted by THAAD or Patriot missile defense systems. They would impose huge losses on South Korea if fired in large numbers, the report said.

A report published on the Chosun Ilbo website in June claimed THAAD may not be able to function properly in an emergency due to power shortages in Seongju, severely impairing its effectiveness.