A former test rugby captain has joined the chorus of social media users mocking US president Donald Trump for creating the non-existent country "Nambia".

Trump came up with the country - possibly confusing Namibia and nearby Zambia - while speaking to African leaders at a United Nations lunch on Wednesday.

He twice referred to "Nambia" in the span of about 800 words.

"Nambia's health system is increasingly self-sufficient," Trump said.

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He went on to make comments about Africa's "tremendous business potential".

"I have so many friends going to your countries trying to get rich. I congratulate you, they're spending a lot of money," Trump said.

Twitter users have been having a field day with Trump's most recent gaffe - with Namibian test veteran Jacques Burger leading the way.

The 34-year-old loose forward, who represented his country on 41 occasions (including in the 2007 and 2011 World Cups) took to Twitter to educate Trump - not only on the correct name for the southern African country but also on one of the nation's favourite past-times - the humble braai (barbeque).

Burger retired last year after playing over 100 games for the English club Saracens. He also represented neighbouring South Africa's Bulls.