Donald Trump has savaged Japan, one of America's closest allies, stating that if the US is attacked, all Japan would do is "sit home and watch Sony television".

He expressed his frustration that the US is bound by treaty to defend the Asian nation but that if the United States is attacked, the Japanese cannot help because of Article 9, which constitutionally forbids it to send armed forces overseas.

He said that it "could be necessary" for the US to walk away from the treaty, or at least threaten to do so.

At a campaign event in Iowa, Mr Trump also repeated his criticism of countries that do not pull their weight in terms of financial contributions to Nato.