Pyongyang said the missile system, also opposed by China, was part of a joint plot between Seoul and Washington to mount a "preemptive attack on the North", according to a statement on the official KCNA news agency attributed to the National Peace Committee of Korea.
The statement coincided with a a visit by new US Defense Secretary James Mattis to Seoul, where he pledged to deploy THAAD -- the Terminal High Altitude Area Defense system -- to protect US and South Korean troops.
Mattis promised an "overwhelming" response to any attack mounted by North Korea.
"North Korea continues to launch missiles, develop its nuclear weapons program and engage in threatening rhetoric and behavior," Mattis said at a press conference in Seoul on Friday.
Speaking alongside South Korean Defense Minister Han Min-koo, he added: "Any attack on the United States or on our allies will be defeated and any use of nuclear weapons will be met with a response that will be effective and overwhelming."
The North Korean statement said THAAD was part of an offensive strategy between Seoul and Washington, alleging the two had "worked out a plan for a preemptive attack on the North and ... desperately pushed forward it, pushing the situation on the peninsula to the brink of a nuclear war."
On Thursday Mattis said Pyongyang's "provocative behavior" made THAAD necessary.
China again called for the THAAD missile defense system to be halted. In his regular press conference on Friday, Chinese Foreign Ministry spokesman Lu Kang said Beijing "firmly opposed" the new system.
"This has not changed and will not change. Our position is very clear-cut and straight forward," he said.
"We believe that the action by the US and the Republic of Korea will undermine the strategic and security interests of regional countries including China."
Later on Friday, Mattis traveled to Japan, which also feels threatened by North Korea's nuclear and missile programs.
Speaking in Tokyo at a meeting with Japanese Prime Minister Shinzo Abe, Mattis reaffirmed Washington's support. "I want there to be no misunderstanding during the transition in Washington that we stand firmly, 100 percent shoulder to shoulder with you and the Japanese people, Mr. Prime Minister."
Mattis' visit to the region comes at a time of relative quiet from Kim Jong Un's regime, which hasn't tested a ballistic missile since October 20. Earlier in 2016, it fired projectiles at a record rate.
North Korea nevertheless continues to make bellicose statements, including one from leader Kim Jong Un on New Year's Day in which he said his military is on the brink of testing its first intercontinental ballistic missile -- a rocket that could be equipped with nuclear weapons and is powerful enough to reach any part of the United States.
Analysts point out, however, that Kim's bluster is often more for internal consumption
than an actual threat to South Korea and the US, which has 28,000 troops in South Korea.
"He demands from his people and subordinates complete obedience to his leadership, because the country is on the verge of imminent invasion from the US and South Korea," said Seung-Kyun Ko, a professor at Hawaii Pacific University and a former research commissioner in South Korea's Ministry of Foreign Affairs. "He creates cohesion and unity among his people in facing the invasion."