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.
Nonetheless, on Sunday the president made a case that those sanctions were having an effect. He wrote on Twitter that he had spoken with South Korea’s president, Moon Jae-in, and tossed out a new nickname for the North’s leader, Kim Jong-un.
“Asked him how Rocket Man is doing,” President Trump wrote. “Long gas lines forming in North Korea. Too bad!”
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 American effort to halt Mr. Kim’s missile program.
“If North Korea does not have UDMH, it cannot threaten the United States, it’s as simple as that,” said Senator Edward J. Markey, Democrat of Massachusetts, 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 American 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.