Republican presidential front-runner Donald Trump on Sunday doubled down on his criticism of NATO, a cornerstone of US 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 US 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.The billionaire businessman, vying to win his party's nomination for the Nov. 8 presidential election, also said he might halt US purchases of oil from Saudi Arabia and other Arab allies unless they commit ground troops to fight Islamic State or pay the United States to do so."NATO is obsolete," Trump said on ABC's This Week with George Stephanopoulos.The 28-country North Atlantic Treaty Organization was set up in a different era, Trump said, when the main threat to the West was the Soviet Union. It is ill-suited to fighting terrorism and costs the United States too much, he added."We should readjust NATO ... it can be trimmed up and it can be, uh, it can be reconfigured and you can call it NATO, but it's going to be changed," he said.