US President Donald Trump and South Korea's leader Moon Jae-In agreed Monday to remove limits on the payload of the South's missiles, Seoul's presidential office said, as the UN Security Council met to discuss a response to Pyongyang's sixth and most powerful nuclear test.

The two leaders in a phone call "agreed to lift the cap on missile payload of South Korea as an effective countermeasure" against Pyongyang's test on Sunday of what it described as a hydrogen bomb designed for a long-range missile, the presidential office said in a statement.

Seoul was perviously restricted to a maximum warhead weight of 500 kilogrammes (1100 pounds) on its ballistic missiles, according to a bilateral agreement with the United States.