SEOUL — North Korea and Iran are stepping up their joint efforts at producing nuclear warheads and missiles, an Israeli analyst is warning, with the North poised to offer still more assistance as Iran hovers on the brink of producing its first nuclear warhead. The Islamic Republic appears anxious to go nuclear even as Washington and Tehran are talking about a deal for Iran not to produce nuclear warheads.

A long-time Israeli aerospace and missile researcher, Tal Inbar, said the North has been giving “crucial parts” for Iranian missiles ever since the Iran-Iraq war in the 1980s. On the basis of this “longstanding” cooperation, he contended, “if the time comes, Iran would get the nuclear core” for warheads from North Korea.

North Korea’s dealings with Iran parallel its growing role as exporter to Russia of arms and artillery shells that are badly needed to support Russian forces in Ukraine.

The American ambassador to South Korea, Philip Goldberg, talking to Korean reporters at his residence here, accused Russia of “dealing in weaponry and discussions of military cooperation with a regime that has flouted every aspect of international rule of law.”

Mr. Inbar’s remarks about the Iran-North Korea connection come as the North’s leader, Kim Jong-un, ordered more tests of short-range missiles as America and South Korea wound up 11 days of joint exercises with flights of fighter jets and a single American strategic B1B bomber. They were the biggest American-South Korean military exercises in years. 

While Mr. Inbar warned of the escalating relationship between Pyongyang and Tehran, an American academic outlined here a more sanguine view of the North’s nuclear program. 

In the keynote speech at a forum staged by South Korea’s unification ministry, a University of Chicago professor of international relations, John Mearsheimer, said North Korea’s missile program actually was a stabilizing force in the standoff between the two Koreas.

“Many people believe that a nuclear-armed North Korea is a source of instability,“ he said. “They are wrong. A nuclear-armed North Korea is likely to make the Korean peninsula more stable than would otherwise be the case if North Korea had no nuclear weapons.”

Mr. Mearsheimer, author of books on rivalries among great powers, said there’s “little doubt that stability on the Korean peninsula would be even more robust if South Korea had its own nuclear deterrent,” but he said that was “unlikely because of strong U.S. opposition.” Still, he argued, “a survivable North Korean nuclear deterrent and the U.S. nuclear umbrella” provides “abundant stability.” 

What about North Korea’s close ties with Iran — and their dealings in what President George W. Bush in 2002 described as an “axis of evil” that also included Saddam Hussein’s Iraq? Largely Shiite lran exerts strong influence in Iraq through Iraq’s Shia majority. While Iraq survives uneasily with American assistance, Iran-North Korean collusion remains a threat from the Korean peninsula to the Middle East.

It’s for that reason, Mr. Inbar said, that South Korea “is now developing its own version” of the vaunted “iron dome” of missile defense that protects Israel from Iran and Iran-armed militias, including Hamas in Gaza, across Israel’s southern border, and Hezbollah in Lebanon.

“There is a need for early warning,” Mr. Inbar said. “It’s not so easy to intercept missiles” — especially those powered by solid fuel that are ready to fire the moment they’re placed on the launch pad.

Iran, he said, “has been focusing on solid-propelled missiles for years,” while North Korea has only recently been test-firing them, presumably with technological advice from Iran. Liquid-fueled missiles are much easier to detect since they’re fueled on the pad, a process that takes half an hour, time for satellites to see what’s going on.

Obviously, Mr. Inbar said, talking at a briefing arranged by NK Pro, a site in Seoul, there’s mounting concern at Seoul about North Korea in view of the torrent of rhetoric from Mr. Kim. Most recently, the North has conducted exercises complete with simulated nuclear strikes and an invasion of South Korea.

The situation conjures memories of June 1950, when Mr. Kim’s grandfather, Kim Il-sung, sent North Korean troops pouring into the South. “Serious risks to stability on the Korean peninsula remain,” a senior fellow at the Carnegie Endowment on International Peace, Ankit Ponda, said, while Mr. Kim looks for “some sort of coexistence that allows North Korea to retain nuclear weapons.”

At the other end of the “axis of evil,” Mr. Inbar noted that the diameter of Iranian missiles was “the same as the bomb” that Mr. Kim has shown in Pyongyang. “There could be cooperation,” he said, in the face of Iranian denials that it’s aspiring to become the 10th member of the world’s nuclear club, an elite grouping that includes, most recently, North Korea.

As of now, it’s not clear if Mr. Kim is going to order the North’s seventh underground nuclear test — its first since September 2017, when the test of what might have been a hydrogen bomb blew up much of the interior of a mountain, possibly killing 200 people.

Mr. Inbar suspects that Iranians were managing a reactor made by North Korea in Syria that Israeli planes blew up in 2007. “We don’t know if North Korea will conduct a test in the future,” Mr. Inbar said, but “it could be with the cooperation of Iran.”