Zimbabwean Vice President Emmerson Mnangagwa has reportedly thanked President Robert Mugabe's wife Grace for being the "strength" behind his boss.

"We want to thank our mother (Grace). She is the one who cooks and ensures that everything with our President is in good order. When he went out to speak at the UN he was well fed by Amai (mother) and well supported, that’s why he had strength to fight [US President Donald] Trump," Mnangagwa was quoted as saying.

According to News Day, Mnangagwa said this at the Harare International airport, where he welcomed Mugabe back from the United Nations General Assembly in New York on Monday.

Mugabe did not mince words at the United Nations last week about Trump, mocking the US president as the "Giant Gold Goliath".

Addressing the UN General Assembly, where reproaches of other leaders are generally less personal in tone, the 93-year-old veteran leader took Trump to task both on policy and appearance.

'Are we having a return of Goliath to our midst'

"Some of us were," Mugabe said, pausing for emphasis, "embarrassed, if not frightened, by what appeared to be the return of the biblical Giant Gold Goliath".

"Are we having a return of Goliath to our midst, who threatens the extinction of other countries?" he asked, triggering applause in the hall as two junior US diplomats listened expressionless.

"And may I say to the United States president, Mr Trump, please blow your trumpet - blow your trumpet in a musical way towards the values of unity, peace, co-operation, togetherness, dialogue, which we have always stood for and which are well-writ in our very sacred document, the Charter of the United Nations."

Trump stunned longtime UN watchers on Tuesday by threatening from the podium to "totally destroy" North Korea, describing its leader Kim Jong-Un as "Rocket Man".