North Korea recently stopped its missile tests for nearly four months, but since February, it has conducted monthly launchings.
On Monday, the North threatened to retaliate “thousands of times over” against the United States for the latest United Nations sanctions, calling them a “violent infringement” on its sovereignty that only deepened its resolve to strengthen its nuclear arsenal.
In a statement carried by its official Korean Central News Agency, the North said that the United States and “other countries as large” had reacted with “fear” to just two North Korean tests of intercontinental ballistic missiles. That made Pyongyang “more determined that this is the only way we can survive, the only path we can take,” the statement said.
In what appeared to be a blunt warning to the Trump administration, President Moon Jae-in of South Korea on Monday strongly opposed any military actions against the North that could set off war.
“Above all, President Moon emphasized that South Korea can never accept a war erupting again on the Korean Peninsula,” Mr. Moon’s office said in a statement describing a 56-minute phone call with President Trump. “He stressed that the North Korean nuclear issue must be resolved in a peaceful, diplomatic manner through close coordination between South Korea and the United States.”
United States officials said Mr. Moon had requested the call. The White House, in its own statement about the conversation, said the two leaders had “affirmed that North Korea poses a grave and growing direct threat.” Mr. Trump himself described the call on Twitter, praising the weekend vote by the United Nations Security Council in favor of sanctions.
Mr. Moon’s comments followed alarm in South Korea over recent statements from top American officials and politicians saying that military actions remained an option in dealing with the North. Last Tuesday, in comments that were widely reported in South Korea, Senator Lindsey Graham, Republican of South Carolina, said Mr. Trump would not allow Pyongyang to threaten America with a nuclear weapon.
“If there’s going to be a war to stop it, it will be over there,” Mr. Graham told NBC’s “Today” show. “If thousands die, they’re going to die over there — they’re not going to die here. He’s told me that to my face — and that may be provocative, but not really. When you’re president of the United States, where does your allegiance lie?”
Many South Koreans fear that an American attack on the North’s key military sites could begin an armed conflict, or even a full-scale war, that would wreak havoc on the South. The capital, Seoul, a city of 10 million, lies within range of North Korean artillery, rockets and missiles.
On Saturday, the United Nations Security Council imposed the toughest sanctions yet on North Korea, an action that could cost the country $1 billion a year, or about a third of its foreign earnings.
Despite Mr. Ri’s meetings in Manila on Sunday and Monday, none of the diplomats gathered in the Philippine capital even hinted that North Korea had budged on its stubborn pursuit of the ability to send an intercontinental ballistic missile to the United States.
Mr. Ri told his South Korean counterpart Kang Kyung-wha, for instance, that the South’s proposal to improve ties “lacks sincerity,” according to the South’s Yonhap News Agency, Reuters reported.
Since 2006, North Korea has defied a half-dozen Security Council resolutions over its nuclear and missile development, which the North’s leader, Kim Jong-un, has called a necessary, just response to military threats by the United States and South Korea.
China accounts for more than 90 percent of North Korea’s external trade. China has long opposed tough economic sanctions against the North for fear that they could bring down the government and release a flood of refugees. A collapse could also lead to the North’s reunification with the South, putting a close American ally directly on China’s border.
The Trump administration has been trying to get China to rethink that calculus. Beijing’s decision to support Saturday’s resolution as well as a strongly worded rebuke of Mr. Ri by the Chinese foreign minister, Wang Yi, gave some hope that was happening.
“Do not violate the U.N.’s decision or provoke the international society’s good will by conducting missile launching or nuclear tests,” Mr. Wang said he had told Mr. Ri.
Asked Monday whether sanctions would work fast enough to prevent North Korea from acquiring the ability to strike the United States with a nuclear weapon, Mr. Tillerson acknowledged the difficulty.
“I think your point is well taken,” Mr. Tillerson said. “When do these actually have a practical bite on their revenues?”
But he said the sanctions not only sent a stern message to North Korea but were also important signals to China and Russia.
“I think the world is also expressing a view to China and Russia that they do have an expectation that you will do everything you possibly can to help North Korea understand the reality of their future as well and bring them to the negotiating table,” Mr. Tillerson said.
Also Monday, Mr. Tillerson said he had asked Sergey V. Lavrov, the Russian foreign minister, to clarify President Vladimir V. Putin’s demand that the United States reduce its diplomatic mission by 755 employees, part of a series of retaliatory measures the likes of which have not been seen since the height of the Cold War. Last week, President Trump signed legislation imposing sanctions on Russia over its meddling in last year’s American election.
Because of an editing error, an earlier version of this article misidentified the state that Senator Lindsey Graham represents. It is South Carolina, not North Carolina.