North Korea’s miniaturization of nuclear weapons has gone from essentially the size of a large refrigerator a decade ago to something small enough to fit inside its ballistic missiles, according to experts.

The U.S. believes that North Korea mastered miniaturizing nuclear devices on long-range missiles “months ago,” a government source told CNBC on Tuesday. That news was not entirely unexpected — even six years ago South Korean intelligence was aware the hermit regime was becoming more proficient in downsizing its nuclear weapons — but it does mean a significant step for Pyongyang.

NBC News on Tuesday confirmed a Washington Post report about “the U.S. belief that North Korea reached this milestone.” The Post indicated the assessment was from a Defense Intelligence Agency analysis only completed last month and also said the U.S. had calculated that the North had “up to 60 nuclear weapons” in its arsenal. That’s about three times what some experts previously thought.

The DIA and Pentagon declined comment when contacted by CNBC

After North Korea’s second Hwasong-14 intercontinental ballistic missile test last month, defense experts said it appeared the regime’s long-range missile had the range to reach half, if not most, of the continental United States. The North followed it up with propaganda and threats against the U.S. mainland, and Wednesday it claimed to be “carefully examining” a strike on Guam, a U.S. territory in the Pacific with an estimated 6,000 U.S. troops as well as bomber and fighter aircraft.

Regardless, NBC News said its source who was briefed on the nuclear assessment indicated that, despite Pyongyang having the ability to make a small nuclear weapon, it “does not mean that North Korea has fielded a nuclear-tipped intercontinental ballistic missile.”

In February, researchers at Sejong Institute, a South Korean think tank, concluded that the North was capable of producing more nuclear weapons and with smaller quantities of material.

Also, as far back as 2011, South Korean intelligence took notice that Pyongyang was capable of building smaller nuclear weapons and even looking to use missiles to carry chemical or biological warfare agents.

“They (nuclear weapons) have to be light and small to go further,” said Ramesh Thakur, director of the Center for Nuclear Non-Proliferation and Disarmament at Australian National University’s Crawford School of Public Policy and co-convenor of the Asia-Pacific Leadership Network for Nuclear Non-Proliferation and Disarmament. “Clearly, the North Koreans have had their own scientists working at this at breakneck speed.”

The regime’s leader, Kim Jong Un, boasted about the feat early last year and Pyongyang’s state media showed pictures of a round metallic object that may have been part of a miniaturized warhead capable of fitting in the tip of a ballistic missile.

Still, the question is whether a nuclear-armed ICBM can also survive “the technical rigors of reentering from space,” said Thakur. “The ICBM comes in at very high speed and has to survive extreme temperatures and extreme vibrations.”

Indeed, a former United Nations missile inspector said last month the second test of the Hwasong-14 missile on July 28 may have failed on reentry and so it may require more testing.

Michael Elleman, a consulting senior fellow for missile defense at the International Institute for Strategic Studies, made the assessment after analyzing video captured from a rooftop camera in Japan that appeared to show the ICBM reentering the atmosphere, then breaking up and destroying the reentry vehicle — a part of the missile critical to the nuclear weapon’s survival.

Bruce Klingner, a former CIA Korean branch chief and now senior research fellow for Northeast Asia at the Heritage Foundation’s Asian Studies Center, said the time that it took India and Pakistan to test nuclear weapons to the time when people “ambiguously assessed” that they had miniaturized warheads was a shorter overall time span than it’s taken since North Korea’s first nuclear test in 2006. North Korea conducted its fifth nuclear blast last September.

By comparison, China and the former Soviet Union conducted around four nuclear tests apiece before experts concluded they too had the ability to miniaturize nuclear warheads on long-range ballistic missiles. The U.S. government today also believes China obtained information through espionage in the 1980s and as late as the 1990s about U.S. nuclear secrets and miniaturization of bombs that allowed it to shrink the size of weapons and make them even more powerful.

According to Klingner, experts assess that North Korea already miniaturized the nuclear warhead for the Nodong medium-range ballistic missile, which has a range to reach key U.S. allies in East Asia, including South Korea and Japan.

“An ICBM is more difficult,” he said. “Most folks think North Korea hasn’t demonstrated a [reentry vehicle for the ICBM], but they could do it tomorrow or they could do it next year.”