US President Donald Trump is questioning why Iran has not yet "capitulated" in the face of Washington's military build-up in the Middle East, the US president's special envoy Steve Witkoff has said. Witkoff told Fox News on Saturday that Trump was "curious" about Iran's position after he had warned of a limited military strike if a deal was not reached on Tehran's nuclear programme. The US and its European allies suspect Iran of moving towards making a nuclear weapon, which it denies. Within Iran, anti-government protests were staged at several universities over the weekend - the first rallies on such a scale since January's deadly crackdown by the authorities, which saw thousands killed. In his interview with Fox News, Witkoff said: "I don't want to use the word 'frustrated'... because he [Trump] understands he's got plenty of alternatives, but he's curious as to why they haven't...…