China and Russia should welcome the decision to deploy the U.S. THAAD missile defense system in South Korea because it helps lessen the need for Seoul to seek other defense options, including development of its own nuclear weapons, a U.S. expert said Wednesday.

Richard Weitz, director of the Center for Political-Military Analysis at the Hudson Institute in Washington, made the point in an article, titled "Better THAAD than Dead," arguing that South Korea has ample reasons to shore up defense and nuclear weapons development could be an option.

Should the South go nuclear, Japan will follow suit, a scenario that is not in China's interest, he said.

"China and Russia should welcome THAAD, because it alleviates the need for South Korea or Japan to pursue other defense options, which could include the development of nuclear weapons," Weitz said in the article.

China has strongly protested the decision to place a Terminal High Altitude Area Defense unit in South Korea, claiming the system could be used against the country, despite repeated assurances from Seoul and Washington that the battery is aimed only at defending against North Korean missile threats.

Weitz said that many in South Korea and Japan now worry about the strength of U.S. security commitment. Recent developments have undermined U.S. credibility, from President Barack Obama's failure to enforce his 'red line' warning against the use of chemical weapons in Syria, to Donald Trump's presidential nomination, he said.

"Some members of South Korea's governing conservative Saenuri Party now openly call for the acquisition of nuclear weapons, believing that this will deter a North Korean attack and prompt China to increase pressure on its client to roll back its weapons programs," Weitz said.

"If South Korea develops a nuclear arsenal, Japan is likely to do the same, especially given China's aggressive pursuit of its claim to the Japanese-occupied Senkaku Islands," he said. "Japan has an enormous stockpile of separated plutonium and the technical wherewithal to be a 'virtual nuclear power': without having any nuclear weapons on hand, it could quickly develop them if necessary."

But nuclear weapons development by South Korea and Japan would not only put the countries under international sanctions, but it would also "be the start of a new Cold War, or worse," the expert said.

"Against this backdrop, the case for THAAD is clear. The US, South Korea, and Japan should cooperate on missile defense as the first line of regional deterrence, while also fortifying and dispersing vulnerable targets, deepening trilateral intelligence sharing on North Korean threats, and working with the international community to disrupt the North's weapons programs," he said.

"South Korea and Japan should not alienate the US by setting out on their own - no matter what Trump tells them," he said. (Yonhap)