Donald Trump often likes to apply a simple test to audience: are you with me or against me, are you a friend or a foe?

During a rally on Friday, he provoked anger when he asked a Turkish reporter whether he was friend or foe, days after suspected Islamic State suicide bombers killed 43 people at Istanbul's Ataturk Airport.

The exchange came as the presumptive Republican presidential nominee rattled off a string of countries with which he believed the US could strike better deals.

A voice in the crowd in Denver called out: “Turkey.”