Donald Trump has attacked 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.

“You know we have a treaty with Japan, where if Japan is attacked, we have to use the full force and might of the United States,” he said.

“If we’re attacked, Japan doesn’t have to do anything. They can sit home and watch Sony television, OK?”

Mr Trump added that the United States protects Japan, South Korea, Germany, Saudi Arabia and other nations, and “they don’t pay anything near what it costs”.

“It’s got to be a two-way street.”

It was the latest salvo from the Republican nominee, at the end of a week in which he has:

• Insulted the grieving parents of a Muslim American war hero

• Refused to endorse Paul Ryan, the most senior Republican, in his re-election bid

• Made a series of foreign policy gaffes

• Told a mother with a crying baby to leave one of his rallies

• Made an inappropriate joking when accepting a Purple Heart medal from a veteran

• Seen his polling numbers plummet and his campaign reach “crisis” levels

The US and Japan signed the Treaty of Mutual Cooperation and Security on January 19, 1960.

Both countries agreed to assist each other in case of armed attack on territories under Japanese administration. However, Japan cannot currently come to the assistance of the US if it is attacked, because of Article 9, which forbids the maintenance of “land, sea, and air forces”.

There are currently 47,000 US military personnel stationed in Japan, and the alliance with Japan is crucial for America’s Asia-Pacific strategy and security.

Mr Trump threatened Japan that, under his presidency, it could be necessary to “walk” away from the treaty.

“It could be that Japan will have to defend itself against North Korea,” he said.

“You always have to be prepared to walk,” Mr Trump said about getting allies to carry their financial weight.

“I don’t think we’ll walk, I don’t think it’s going to be necessary. It could be, though.”