Current guidelines limit the destructive capabilities of South Korea's own missiles, but Pentagon spokesman Navy Captain Jeff Davis said the restrictions might be altered.

"It is a topic under active consideration here and I would tell you that we would be favorably inclined to do anything which furthers the defensive capabilities of South Korea," Davis told reporters.

"We have the ability to adapt and we will always adapt to the threat as it changes."

The announcement comes a day after President Donald Trump and his South Korean counterpart Moon Jae-In spoke on the phone and agreed the North "poses a grave and growing direct threat," according to a White House statement.

North Korea last month conducted two intercontinental ballistic missile tests, after which Kim boasted that he could now strike any part of the United States.

The Pentagon did not immediately provide additional information on the warhead limits set forth in current guidelines.

An official said the guideline revisions were being coordinated in conjunction with the State Department.

The UN Security Council unanimously approved tough new sanctions against North Korea over the weekend that could cost Pyongyang $1 billion a year.