US President Donald Trump. Photo: AFP

An unknown projectile hit a cargo ship in the Strait of Hormuz, a key point of tension in the US-Iran conflict, on Tuesday as President Donald Trump insisted the waterway could reopen within hours.

Speaking from the White House on Monday, the US leader continued his now familiar combination of bullish optimism about diplomacy and bellicose threats, warning that this was Iran’s “last chance before decapitation”.

Tehran and Washington have been at war since February 28, when the US and Israel launched surprise attacks on Iran. Despite a ceasefire and a preliminary deal in the months since, diplomacy has failed to end the war.

The Strait of Hormuz, a key conduit for global oil and gas supplies, has been the spark for repeated returns to fighting.