The conservative parties, including the ruling Liberty Korea Party, however, welcomed the delivery of THAAD parts, citing growing security challenges from North Korea.

"(The government) has completely disregarded public calls and demands from political circles with regard to the THAAD deployment," Youn Kwan-suk, the spokesman for the Democratic Party, said in his commentary.

"From the clandestine push for the deployment to the process of making the final decision on the installation, the government has unilaterally pressed ahead with it without any consultations with citizens," he added.

Seoul's military authorities announced earlier in the day that the US military has started dispatching part of its THAAD battery, a core element of Washington's multilayered global missile shield, to counter Pyongyang's growing nuclear threats. The announcement came a day after the North fired off four ballistic missiles toward the East Sea.

The deployment process was initiated at a politically delicate time when the nation is awaiting a Constitutional Court ruling on whether to uphold the parliamentary impeachment of President Park Geun-hye over a corruption scandal.

The main opposition party argued that the government might have rushed to install THAAD to help forge a public sentiment more favorable for the impeached president and her conservative party, which have long considered security as its forte.

The minor People's Party also berated the government for the deployment, calling for an immediate stop to the installation process.

"We have doubts over whether there is the need for the hurried deployment even though North Korea remains unstable," Kim Kyung-jin, the party's spokesman, said in his commentary. "Our stance remains unchanged that the THAAD deployment requires parliamentary consent."

The THAAD deployment is underway even though preparatory procedures such as the construction of the installation facilities in the southern county of Seongju, 296 kilometers south of Seoul, have yet to be completed.

Also, Seoul and Washington are still in the process of discussing legal matters related to the THAAD operation here under the Status of Forces Agreement that governs the legal status of US military assets here.

Meanwhile, the Liberty Korea Party hailed the ongoing deployment process as the "right" decision.

"We welcome the right decision to deploy THAAD when North Korea threatens the security of the Republic of Korea and Northeast Asia by firing as many as four ballistic missiles yesterday (Monday)," Kim Sung-won, the spokesman for the ruling party, said in his commentary.

Oh Shin-hwan, the spokesman for the Bareun Party, also hailed the arrival of THAAD battery components, saying "security cannot be traded for anything."

Undeterred by the opposition parties' offensive, the presidential office Cheong Wa Dae defended the THAAD deployment as a "self-defense" measure against tangible North Korean threats. (Yonhap)