Donald Trump said providing support to Montenegro - both of which are members of the North Atlantic Treaty Organisation (Nato) – could mean starting “World War III”.

Fox News host Tucker Carlson asked the president why his son should “go to Montenegro to defend it from attack”, as a way of criticising the organisation’s central philosophy: an attack on one member is an attack on all of them.

“I understand what you’re saying, I’ve asked the same question,” the president said, adding: “You know, Montenegro is a tiny country with very strong people...They are very aggressive people, they may get aggressive, and congratulations, you’re in World War III. But that’s the way it was set up. Don’t forget, I just got here a little more than a year and a half ago.”

The president has routinely criticised the military alliance begun in the years after the Second World War in order to protect European nations from the threat of a Soviet invasion. Montenegro, a small Balkan country on the Adriatic Sea, joined the alliance last summer.

During his first meeting at Brussels headquarters, Mr Trump did not endorse Article 5 of the alliance’s charter, the item that outlines mutual defence.

The only time that article has been invoked was when the US was attacked on September 11 2001.

Trump appears to shove fellow NATO leader

Since joining Nato, Montenegro has sent troops to the ongoing war in Afghanistan, which began shortly after the attacks.

At this year’s meeting, Mr Trump hit out at members for not contributing a fair share of money towards the defence budget.

Nato leaders gather for summit: in pictures Show all 10 1 /10 Nato leaders gather for summit: in pictures Nato leaders gather for summit: in pictures World leaders gather for Nato family photo Getty Images Nato leaders gather for summit: in pictures British Prime Minister Theresa May arrives EPA Nato leaders gather for summit: in pictures President Donald Trump talks to British Prime Minister Theresa May EPA Nato leaders gather for summit: in pictures U.S. President Donald Trump, left, talks to British Prime Minister Theresa May during a family photo at a summit of heads of state and government at NATO headquarters in Brussels Wednesday, July 11, 2018. NATO leaders gather in Brussels for a two-day summit to discuss Russia, Iraq and their mission in Afghanistan. (AP Photo/Markus Schreiber) Markus Schreiber AP Nato leaders gather for summit: in pictures US President Donald Trump and Nato Secretary General Jens Stoltenberg attend a bilateral breakfast ahead of the Nato Summit in Brussels Reuters Nato leaders gather for summit: in pictures President Donald Trump, right, gestures as he speaks to the media prior to his bilateral breakfast with Nato Secretary General Jens Stoltenberg, left AP Nato leaders gather for summit: in pictures President Donald Trump, US Secretary of Defence James Mattis and US Secretary of State Mike Pompeo at the bilateral breakfast with Nato Secretary General Jens Stoltenberg REUTERS Nato leaders gather for summit: in pictures President Donald Trump and Mike Pompeo, left, are reflected in a mirror as they attend at a breakfast meeting AFP/Getty Images Nato leaders gather for summit: in pictures President Donald Trump speaks during the bilateral breakfast EPA Nato leaders gather for summit: in pictures President Donald Trump gestures during the bilateral breakfast EPA

“Many countries in Nato, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made. Will they reimburse the US?,” he tweeted, not fully understanding how the organisation operates.

Countries do not owe the US any money as part of their membership in Nato. Contributing a percentage of a country’s GDP is simply a commitment to do so to Nato, not to countries such as the US that have been paying their dues.

The US president and the country with a population of less than 650,000, have had a somewhat strange interaction before this most recent one.

At last year’s Nato summit, it appeared Mr Trump may have shoved Montenegrin Prime Minister Dusko Markovic out of the way to get to the front of the group of world leaders walking to pose for pictures.

Fourteen people are on trial in Montenegro for a plot to kill Mr Markovic and bring a pro-Russia party into power. The government claimed the attempted coup was part of a plan to stop the former Soviet Union country from joining Nato.