He added, "We are well aware how much effort President Trump has put into leading China" to more actively rein in North Korea, emphasizing the importance of the South Korea-U.S. alliance in that process. "The Unification Ministry has considered options on this internally but nothing has been decided yet".
He added that this is in line with Moon's statement at a Sunday meeting of the South Korean National Security Council that dialogue is possible only if and when there is a change in North Korea's behavior.
The North has vowed to develop a missile mounted with a nuclear warhead that can strike the mainland United States. The North defends its weapons programs as necessary to counter US hostility.
The Security Council has adopted six increasingly tougher sanctions resolutions against North Korea.
Moon, who has been an advocate of engagement with the North, said during his campaign last December that Pyongyang would be the first destination he would visit if elected president. After taking office, he said that he wants to pursue dialogue as well as pressure to stop the North's weapons programs.
China for its part has been infuriated by the USA deployment of an advanced Terminal High Altitude Area Defense (THAAD) anti-missile system in South Korea, saying it was a threat to its security and would do nothing to ease tension with Pyongyang.
Moon's envoy to the United States, South Korean media mogul Hong Seok-hyun, left for Washington early on Wednesday.More news: Celtics and Lakers Big Winners on Lottery Night
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U.S. President Donald Trump gestures as he walks on the South Lawn of the White House upon his return to Washington, U.S., May 17, 2017.
"I said there is a controversy about procedural problems in the course of the deployment, and I talked about the need for this issue to be discussed at the National Assembly", Hong told reporters at the Korean War Veterans Memorial after paying tribute to US soldiers who served during the 1950-53 Korean War.
China has strongly opposed THAAD, saying it can spy into its territory, and South Korean companies have been hit in China by a nationalist backlash over the deployment.
U.S. Ambassador Nikki Haley said on ABC television that the United States has been working well with China, Pyongyang's closest ally, and she raised the possibility of new sanctions against North Korea, including on oil imports.
North Korea on Sunday launched what appeared to be its longest-range ballistic missile yet, claiming it was capable of carrying a "heavy nuclear warhead" in a test aimed at bringing the U.S. mainland within reach. She also added that any country that doesn't implement the United Nations sanctions is supporting Pyongyang's actions.
"This is not play time.we are going to tighten the screws on this", Haley told reporters as the UN Security Council headed into an urgent meeting to discuss Pyongyang's most recent ballistic missile launch on Sunday.