Republican presidential front-runner Donald Trump on Sunday doubled down on his criticism of NATO, a cornerstone of U.S. foreign policy for decades, and called for the alliance's overhaul days before world leaders convene in Washington.

President Barack Obama will host the Nuclear Security Summit on Thursday and Friday with 56 delegations in attendance. While preventing nuclear terrorism will headline the discussions, Trump's views could be a topic as well, particularly behind the scenes.

In another sharp departure from historic U.S. policy, Trump said in an interview published on Sunday by The New York Times that he would consider letting Japan and South Korea build their own nuclear weapons, rather than rely on America for protection against North Korea and China.