A decision to postpone the full deployment of the Terminal High Altitude Area Defense (THAAD) system pending a review of its environment impact was a domestic measure to ensure a democratic process, Chung Eui-yong told a news briefing.
U.S. ally South Korea said on Wednesday it would hold off on installing remaining elements of the THAAD system, which China strongly objects to, until the environmental study was completed.
"The decision to introduce THAAD was made to protect South Korea and the U.S. forces in South Korea from a growing threat from the North. We won't take this decision lightly," Chung said.
During his successful campaign for a May election, South Korean President Moon Jae-in promised to review the THAAD deployment decision, made by the government of his conservative predecessor, Park Geun-hye.
China says the system's powerful radar can penetrate deep into its territory, undermining its stability and unsettling the regional balance. China also says the system does nothing to deter North Korea.
The United States and South Korea have said the THAAD system is only aimed at defending against North Korean missiles.
Two launchers of the full six-launcher THAAD battery, as well as its radar, have been installed at a site on what used to be a golf course near the southeastern city of Seongju.
Those elements will stay in place, South Korea has said.
On Wednesday, Moon's office said the installation of four additional launchers would be halted during the environmental assessment, which may take well over a year, according to a senior administration official.
Chung said South Korea would discuss measures to strengthen its alliance with the United States and address the growing threat from North Korea's weapons programmes during a summit between Moon and U.S. President Donald Trump set for late June.
US Under Secretary of State Thomas Shannon will visit Seoul next week to discuss the summit agenda, the foreign ministry said on Friday. No date has been set.
North Korea has conducted two nuclear tests and numerous tests of various missiles since the beginning of last year, in defiance of U.N. sanctions.
It tested a new type of land-to-sea missile off its east coast on Thursday.
Leader Kim Jong Un has spoken of his aim to develop an intercontinental ballistic missile capable of hitting the mainland United States with a nuclear weapon.
(Reporting by Christine Kim and Soyoung Kim; Editing by Robert Birsel)
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