US President Donald Trump has agreed with South Korean President Moon Jae-in to revise a joint treaty capping the development of the South's ballistic missiles, amid a standoff over North Korea's missile and nuclear tests. <br/><br/>The White House said, Mr Trump also gave "conceptual" approval to the purchase by South Korea of billions of dollars of US military hardware. South Korea wants to raise the missile cap to boost its defences against reclusive North Korea, which is pursuing missile and nuclear weapons programmes in defiance of international warnings and UN sanctions.