Secretary of State Rex Tillerson on Friday called on Russia and China to sever their economic pipeline to North Korea, and said Pyongyang must “earn its way back” to negotiations by ceasing missile tests for a sustained period.

Speaking at the United Nations Security Council in a session called for foreign ministers, Tillerson backpedaled from a position he outlined earlier this week. In New York, he did not repeat an assertion he made Tuesday that the United States has no preconditions to starting talks. That remark was undercut by the White House, which said there had been no change in position.

Tillerson said Friday that any talks must be preceded by a “sustained cessation of North Korea’s threatening behavior,” and the United States will continue to ask countries to step up their efforts to isolate the regime and starve it of cash by adhering to sanctions.

“North Korea must earn its way back to the table,” he said. “The pressure campaign must, and will, continue until denuclearization is achieved. We will, in the meantime, keep our channels of communication open.”

North Korea’s ambassador to the U.N., Ja Song Nam, joined the Security Council session in a rare appearance. Under U.N. rules, countries being discussed at the Security Council are invited to attend, but North Korean diplomats usually boycott.

Reading from written remarks, he said North Korea’s nuclear weapons program was defensive and needed to ward off a U.S. invasion to oust the regime. He accused the Security Council of acting as a “tool” of the United States, which he said used its own stockpile of weapons to deploy as “nuclear blackmail.”

“If anyone is to blame for it, the U.S. is the one who must be held accountable,” he said.

But Tillerson and every other diplomat who spoke at the Security Council expressed disapproval if not alarm over North Korea’s defiance of sanctions aiming to get it to abandon its program. North Korea has accelerated the pace of its missile program, launching 40 ballistic missiles and conducting three nuclear tests over the past two years. Each test exhibited technological improvements.

“The international community remains firm in our determination that we will not accept a nuclear North Korea,” Tillerson said.

That position could by itself preclude any negotiations. Pyongyang has been adamant that it will never even consider abandoning its nuclear weapons program.

“The DPRK has a choice,” said Tillerson, using the acronym for North Korea’s official name, the Democratic People’s Republic of Korea. “It can reverse course, give up its unlawful nuclear weapons programs, and join the community of nations, or it can continue to condemn its people to poverty and isolation.”

If North Korea fires a nuclear missile at us, how would we stop it?

The Security Council meeting was called amid rising concerns the world is on the brink of nuclear conflict. North Korea has boasted its missiles are now capable of striking anywhere in the continental United States. The Trump administration has responded by saying it is prepared for a military conflict, if necessary, even while Tillerson continues to try to isolate North Korea diplomatically and economically by getting more countries to fully implement strict U.N. sanctions.

“The worst possible thing that could happen is for us all to sleepwalk into a war that might have very dramatic circumstances,” U.N. Secretary General António Guterres said Thursday.

Attending the Security Council meeting, Guteres said North Korea’s nuclear program had provoked the most dangerous situation in the world today, and shown a blatant disregard for U.N. sanctions. He said the United Nations could help promote communications.

“We must do everything we can to reach that objective — and avoid a level of danger that would be unpredictable in its trajectory and catastrophic in its consequences,” said Guterres.

In Russia, President Vladi­mir Putin warned Thursday that the United States should not further aggravate North Korean leader Kim Jong Un, whom President Trump has derided several times by referring to him mockingly as “Little Rocket Man.”

Tillerson alluded to the possibility of military action but quickly followed up by saying the goal remains a diplomatic solution to the standoff.

“We have been clear that all options remain on the table in the defense of our nation, but we do not seek, nor do we want, war with North Korea,” he said. “The United States will use all necessary measures to defend itself against North Korean aggression, but our hope remains that diplomacy will produce a resolution.”

After calling for an interlude in North Korea’s missile testing, he added, “The United States will not allow the regime in Pyongyang to hold the world hostage.”

Tillerson called on Russia and China to increase pressure on Pyongyang beyond adhering to stringent U.N. sanctions already in place.

“Continuing to allow North Korean laborers to toil in slave-like conditions inside Russia in exchange for wages used to fund nuclear weapons programs calls into question Russia’s dedication as a partner for peace,” Tillerson said. “Similarly, as Chinese crude oil flows to North Korean refineries, the United States questions China’s commitment to solving an issue that has serious implications for the security of its own citizens.”

In a town hall meeting Tuesday with State Department employees, Tillerson said the "pressure campaign" could potentially deny North Korea of $2.5 billion in export revenue from coal, textiles and labor, and imports on fuel and oil. State Department officials say about 90 percent of North Korea's earnings from exports has dried up because of a strict series of sanctions imposed on the country.

In remarks Tuesday at the Atlantic Council, Tillerson said the United States would talk with North Korea without preconditions. But the White House issued a statement a few hours later that appeared to undercut him by saying: “The president’s views on North Korea have not changed. North Korea is acting in an unsafe way, not only toward Japan, China and South Korea, but the entire world.”

His remarks set off a round of recrimination from White House aides, who said he had "not learned his lesson" since Trump rebuked Tillerson for seeking negotiations with Pyongyang. The aides called Tillerson "irrelevant" and predicted he would be gone by early next year.

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