A former US ambassador to South Korea called Monday for the United States to keep strengthening its "extended deterrence" commitment to the Asian ally as he portrayed North Korea's evolving military threats as "far greater" than before.

During a virtual seminar, Harry Harris, who served as US ambassador to Seoul from 2018-2021, made the call as the recalcitrant regime has been doubling down on its nuclear and missile programs under an apparently aggressive nuclear policy stipulated in its constitution.

But Harris, also a retired admiral, said it would be a "mistake" to redeploy US tactical nuclear arms to the Korean Peninsula despite the North Korean security challenge.

"We have to continue to reinforce our extended nuclear deterrence for South Korea," he said at the seminar hosted by the American Enterprise Institute, an US-based think tank. Extended deterrence refers to the US' commitment to using the full range of its military capabilities, including nuclear, to defend its ally.

The visit of the nuclear ballistic missile submarine, USS Kentucky (SSBN-737), to South Korea in July was a "great" demonstration of the US' deterrence commitment, though Washington cannot do it "routinely," he added.

Harris underscored the importance of extended deterrence, saying, "Our allies don't trust the US enough."

"When they lose that trust, whey they believe that you are not going to extend our nuclear extended deterrent to them, that's when they will proliferate and build their own nuclear weapons and you can't blame (them) for that," he said.

He also pointed out the need to reinforce all three legs of the US' "nuclear triad" that consists of nuclear-capable submarines, bombers and intercontinental ballistic missiles, as he touched on China's drive for nuclear capabilities.

"Those that say we only need two parts of the nuclear triad, and that's wrong," he said. "We need all three elements and ground-based, air, submarine-launched capabilities are each important and must be improved," he said.

Commenting on the North Korean challenge, Harris gave an assessment that the regime perfected its targeting through its repeated short-range ballistic missile tests.

He expressed concerns about technological cooperation between Pyongyang and Moscow.

"All I know is what I read in the news, but in exchange for North Korean munitions, Russia will provide technology to enable North Korea to be successful in its space aspirations," he said. "So, they are far better today capability-wise."

But he said that the alliance is also "stronger" than before.

"I am pleased President Yoon (Suk Yeol) placed the U.S-South Korea alliance as a center point of his foreign policy, and that means a return to joint military exercises," he said, expressing his expectation that the alliance will be better in meeting the North Korean threat. (Yonhap)