Donald Trump agreed with South Korean President Moon Jae-in to revise a joint treaty capping the development of the South's ballistic missiles, Moon's office said on Saturday, amid a standoff over North Korea's missile and nuclear tests.

M Trump also gave "conceptual" approval to the purchase by the South of billions of dollars of US military hardware, the White House said.

The South wants to raise the missile cap to boost its defences against the reclusive North, which is pursuing missile and nuclear weapons programmes in defiance of international warnings and UN sanctions.

"The two leaders agreed to the principle of revising the missile guideline to a level desired by South Korea, sharing the view that it was necessary to strengthen South Korea's defence capabilities in response to North Korea's provocations and threats," South Korea's presidential Blue House said.