Donald Trump agreed to sell South Korea billions of dollars in US arms amid the standoff over North Korea's missile and nuclear tests, the White House said.
The US president also approved that Seoul could review a joint treaty which places a cap on the development of its ballistic missiles – meaning the strike distance and force of the weapons could be increased, South Korea's presidential Blue House said.
The plan to bolster South Korea’s defences came after a request by South Korean President Moon Jae-in to Mr Trump.
The South wants to raise the missile cap to boost its defences against the 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.
Impoverished North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.
North Korea sharply raised regional tension this week with the launch of its Hwasong-12 intermediate-range ballistic missile that flew over Japan and landed in the Pacific.
That followed the test launch of two long-range ballistic missiles in July in a sharply lofted trajectory that demonstrated a potential range of 10,000 km (6,000 miles) or more that would put many parts of the US mainland within striking distance.
North Korea has been working to develop a nuclear-tipped missile capable of hitting the United States and has recently threatened to land missiles near the U.S. Pacific territory of Guam.
South Korea's development of its ballistic missiles is limited to range of 800 km (500 miles) and payload weight of 500 kg (1,100 pounds) under a bilateral treaty revised in 2012.
South Korea has said it wants to revise the agreement to increase the cap on the payload.
The two countries agreed to the cap as part of a commitment to a voluntary international arms-control pact known as the Missile Technology Control Regime, aimed at limiting the proliferation missiles and nuclear weapons.
The two leaders pledged to continue to apply strong diplomatic and economic pressure on North Korea and to make all necessary preparations to defend against the growing threat by the North, the White House said.
The White House did not mention the voluntary bilateral agreement but said the two leaders agreed to strengthen their defence cooperation and South Korea's defence capabilities.
Trump “provided his conceptual approval of planned purchases by South Korea of billions of dollars in American military equipment”, the White House said.
Mr Trump, who has warned that the U.S. military is “locked and loaded” in case of further North Korean provocation, reacted angrily to the latest missile test, declaring on Twitter that “talking is not the answer” to resolving the crisis.
North Korea defends its weapons programmes as necessary to counter perceived US aggression, such as recent air manoeuvres with South Korean and Japanese jets.
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