Intelligence officials believe that the North’s program has advanced to the point where it is no longer as reliant on outside suppliers, and that it may itself be making the deadly fuel, known as UDMH.

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When North Korea launched long-range missiles this summer, and again on Friday, demonstrating its ability to strike Guam and perhaps the U.S. mainland, it powered the weapons with a rare, potent rocket fuel that U.S. intelligence agencies believe initially came from China and Russia.

The U.S. government is scrambling to determine whether those two countries are still providing the ingredients for the highly volatile fuel and, if so, whether North Korea’s supply can be interrupted, either through sanctions or sabotage. Among those who study the issue, there is a growing belief that the United States should focus on the fuel, either to halt it, if possible, or to take advantage of its volatile properties to slow the North’s program.

But it may well be too late. Intelligence officials believe that the North’s program has advanced to the point where it is no longer as reliant on outside suppliers, and that it may itself be making the deadly fuel, known as UDMH, for unsymmetrical dimethyl hydrazine. Despite a long record of intelligence warnings that the North was acquiring both forceful missile engines and the fuel to power them, there is no evidence that Washington has ever moved with urgency to cut off Pyongyang’s access to the rare propellant.

Classified memos from both the George W. Bush and Obama administrations laid out, with what turned out to be prescient clarity, how the North’s pursuit of the highly potent fuel would enable it to develop missiles that could strike almost anywhere in the continental United States.

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In response to inquiries from The New York Times, Timothy Barrett, a spokesman for the director of national intelligence, said that “based on North Korea’s demonstrated science and technological capabilities — coupled with the priority Pyongyang places on missile programs — North Korea probably is capable of producing UDMH domestically.”

Some experts are skeptical that the North has succeeded in domestic production, given the great difficulty of making and using the highly poisonous fuel, which in far more technically advanced nations has led to giant explosions of missiles and factories.

In public, at least, the Trump administration has been far more focused on ordinary fuels — the oil and gas used to heat homes and power vehicles. The United States has pushed to cut off those supplies to the North, but it settled last week for modest cutbacks under a United Nations resolution.

But inside the intelligence agencies and among a few on Capitol Hill who have studied the matter, UDMH is a source of fascination and seen as a natural target for the U.S. effort to halt Kim’s missile program.

“If North Korea does not have UDMH, it cannot threaten the United States, it’s as simple as that,” said Sen. Edward J. Markey, D-Mass., who sits on the Senate Foreign Relations Committee. “These are the issues that the U.S. intelligence community has to answer: from which countries they receive the fuel — it’s probably China — and whether North Korea has a stockpile and how big it is.”

Today, the chemical is made primarily by China, a few European nations and Russia, which calls it the devil’s venom. Russia only recently resumed production of the fuel, after Western supplies were cut off over its annexation of Crimea.

But the Russians are leery of the propellant: It triggered the worst disaster of the Space Age, in 1960, when scores of Soviet workers and spectators died during a test firing of one of Moscow’s early intercontinental ballistic missiles.

The United States no longer produces the fuel — NASA warned of its toxic and explosive dangers as early as 1966, producing a video that opens with a spectacular explosion. Long ago, the U.S. nuclear fleet turned to more stable solid fuels, a move the North Koreans are now trying to replicate. But it may be a decade, experts say, before the North masters that technology to power intercontinental missiles.

Twice — in 2012 and 2014 — the fuel was included in U.N. Security Council lists of prohibited export items. Experts say few paid attention to that fine print.

“All sorts of things banned for export to North Korea find their way in,” said Vann H. Van Diepen, a former State Department official who was at the center of many U.S. efforts to control the spread of weapons of mass destruction.

But the public and involuntarily public record of U.S. efforts to track North Korea’s progress shows a growing concern dating back a decade that the North was obtaining Russian-designed engines to power its missiles, and the fuel to pour into them. A memo designated “secret” and signed in October 2008 by Condoleezza Rice, then the secretary of state, warned allies that the North had obtained an engine powered by UDMH that “represents a substantial advance in North Korea’s liquid propellant technology,” adding that it “allows North Korea to build even longer-range missiles.”

The memo, which was included in documents later released by WikiLeaks, was evidence of early efforts to get countries that had signed the Missile Technology Control Regime to keep such technologies out of the hands of North Korea, Iran and other nations.

The missile launch that took place on Friday, in which the projectile was lofted over northern Japan, was from one of those mobile launchers, fueled by UDMH, spy satellites showed.

The North’s growing dependency on the fuel was reinforced after a military parade in late 2010, when Pyongyang unveiled an intermediate-range missile known as the Musudan. Most of its flight tests failed, some in enormous fireballs.

Federal officials, congressional aides and rocket scientists say emerging clues suggest that, over the years, Pyongyang obtained the fuel, its precursors, its secret formula and its manufacturing gear from China, the North’s main trading partner. Beijing still uses UDMH to loft satellites and warheads and has long exported the toxic substance around the globe.