US Republican presidential front-runner Donald Trump on Saturday continued his criticism of NATO, a cornerstone of US foreign policy for decades, and called for the alliance's overhaul days after world leaders convene in Washington."When I talk about NATO we're talking about tremendous amounts of money that we put in. Now we're defending countries and we're not getting reimbursed anywhere near the cost of doing it. Now you know that might have been OK 30 years ago when we were different and it might have been OK 25 years ago and 40 years ago and it might have been but you know what it's not ok anymore folks we can't do it. We can't do it.," Trump said.In another sharp departure from historic US policy, Trump said 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.