The US military has confirmed it is facing a shortage of ammunition, six months into the war with Iran. That's despite President Donald Trump insisting the country has "virtually unlimited" stocks. The Pentagon watchdog says munitions worth more than US$22 billion were used in the first four months of the conflict. The warning comes as Iran's new security chief ruled out negotiations with the US to dial down tensions. The arms shortage is raising questions over how long Washington can sustain the fight. Nick Harper reports from Washington DC.