Siegfried Hecker, a professor emeritus at Stanford University and renowned nuclear scientist with profound knowledge about North Korea's nuclear program, speaks during a press conference at Ewha Womans University in Seoul, Tuesday. Yonhap

North Korean leader Kim Jong-un has made concerning "strategic" changes in the regime's decadeslong effort of seeking normalization with the United States, turning to Russia and China for cooperation instead, an American expert said Tuesday.

Siegfried Hecker, an emeritus professor at Stanford University known for his direct experience with Pyongyang's nuclear program, made the remarks during a press conference prior to his lecture at Ewha Womans University in Seoul, stressing that the recent link-up of Russia and North Korea is "really serious and significant."

"I think Kim Jong-un in the past year and half or so has made a fundamental strategic decision to give up a 30-year effort of all three Kims to seek normalization with U.S., and instead he turned back to China and Russia," he said. "If that's the case, that's what we have to worry about."

The nuclear expert added that he "worries much more" about Pyongyang's growing cooperation with Moscow and Beijing than the ongoing Israel-Hamas war.

Hecker is considered a top expert on North Korea's nuclear program. He is known for having had a firsthand look at North Korea's uranium-enrichment facility during his 2010 visit to the Yongbyon complex.

Hecker also called it "a very bad idea" for South Korea to develop its own nuclear weapons, saying its alliance with the U.S. will better counter North Korean threats.

"I believe it's a very bad idea for South Korea to develop its own nuclear program, and the main reason for that in my opinion is it will make the Korean Peninsula more dangerous," he said.

"The North Korean nuclear weapons should not be responded by the South, but by the combination of the alliance with the U.S. and with South Korea together," he added.

When asked whether the U.S. government will send its troops to defend South Korea as it did during the 1950-53 Korean War, he stressed the two countries are "close allies."

Hecker added that he believes the U.S. will "come and help defend" South Koreans against any external threats.

On sanctions, Hecker said sanctions on North Korea have not had the "right effect" of stopping the regime's nuclear program and may have instead increased its dependence on China.

"The bottom line is that North Korea has a very threatening nuclear program in spite of the sanctions, so I would say no, the sanctions have not been effective," he said. "For the most part, in my opinion, sanctions have actually worked the other way around, and that is, with sanctioning the economy, I believe it's moved North Korea more towards China." (Yonhap)