President Moon Jae-in is considering making a public statement to explain the government's decision to deploy additional THAAD launchers in the country. "Is it inevitable? Nothing is inevitable", Trump told reporters.
The official also refuted criticism that the administration has been inconsistent on the THAAD deployment.
On Sunday, North Korea had claimed to have detonated a hydrogen bomb that it said can be mounted on an intercontinental ballistic missile, five days after it launched a ballistic missile that flew over Japan.
"Any new actions taken by the worldwide community against the DPRK should serve the goal of curbing the DPRK's nuclear and missile programmes, while at the same time be conducive to restarting dialogue and consultation", he said, referring to North Korea by the initials of its official name, the Democratic People's Republic of Korea.
That rollout was "temporarily" completed on Thursday, a spokesman for South Korea's defence ministry said, adding it was necessary to counter increased threats from North Korea.
Beijing says the system's powerful radars will be able to monitor flights and missile launches deep inside north-eastern China.
A finding by Gallup Korea revealed that South Korean were increasingly doubtful about war compared to how they felt in June 2007 - nine months after Pyongyang conducted its first nuclear test. There are now two THAAD launchers in operation at the Seongju base.
South Korea has since been protected by the United States' nuclear umbrella, which ensures the South will be protected with nuclear weapons when in need.
In a written statement released late Friday evening President Moon explained that adding more launchers to the contentious high-tech missile defense system was the best choice his government could make considering an unprecedented security threat from North Korea.
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The signs read " We denounced the deployment of an advanced USA missile defense system, THAAD".
China's opposition to the THAAD deployment has sent formerly strong relations with South Korea into a tailspin, cutting deeply into the crucial economic relationship between the two countries.
Geng told reporters that China had complained to the USA and South Korea and urged them to "take seriously the security concerns and interests of China and other regional countries".
Seoul's Unification Ministry spokeswoman Eugene Lee said Friday that Pyongyang could potentially conduct its next ICBM test this weekend or around October 10, another North Korean holiday marking the founding of its ruling party.
Chinese President Xi Jinping has called for tougher sanctions against North Korea after he spoke by phone with and German Chancellor Angela Merkel.
Japanese F-15 fighter jets on Saturday conducted an air exercise with U.S. B1-B bombers in the skies above the East China Sea, Japan's Air Self Defence Force (ASDF) said. The US and South Korea should "immediately stop the deployment process and withdraw relevant equipment", Geng said.
But neither side appears willing to budge.
The liberal Moon took office in May calling for diplomacy with Pyongyang but the escalation in weapons tests has been the North's only response.