Former US President Donald Trump says the world is going to be "blown to pieces" if America doesn't get some "smart leadership".

Mr Trump issued the chilling warning during an interview with Sky News Australia host Piers Morgan.

"We have stupid people now running our country," Mr Trump said.

"Our country's in trouble – something has to happen.

"If it doesn't happen, and if we don't have smart leadership, you're going to end up with no world – the world is going to be blown to pieces."