Trump earlier this week said nuclear weapons could help the two U.S. allies boost their ability to deter regional rivals like North Korea.

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“Frankly, it would be catastrophic were the United States to shift its position and indicate we support somehow the proliferation of nuclear weapons to additional countries,” deputy national security adviser Ben Rhodes told reporters at the Nuclear Security Summit in Washington.

The real estate mogul's proposal is just the latest example of how his aggressive foreign policy rhetoric has roiled the presidential campaign.

Trump told The New York Times over the weekend that Japan and South Korea should shoulder more of the expense of defending themselves against North Korea, whose nuclear ambitions have prompted renewed international concern.

“There’ll be a point at which we’re just not going to be able to do it any more,” Trump said of the United States. “Now, does that mean nuclear? It could mean nuclear. It’s a very scary nuclear world.”

Rhodes said Japan and South Korea benefit from the U.S.' "rock-solid security assurances," adding the idea that more countries should obtain nuclear weapons “flies in the face of decades of bipartisan national security doctrine.”

“The entire premise of American foreign policy as it relates to nuclear weapons for the past 70 years has been focused on preventing the proliferation of nuclear weapons to additional states,” he said.

The White House’s comments came after Obama met with Japanese Prime Minister Shinzō Abe and South Korean President Park Geun-hye.

The three leaders said they are united in ridding the Korean Peninsula of nuclear arms. They pledged to enforce new United Nations sanctions against North Korea in response to recent nuclear tests and a rocket launch.

White House officials said the 2016 race didn’t come up in the discussions with the Asian leaders.

“An idea like that is not particularly relevant to the very serious discussions we are having here,” Rhodes said.

The president is expected to meet with Chinese President Xi Jinping on Thursday afternoon to urge North Korea’s traditional ally to follow through on implementing the sanctions.