US President Joe Biden says addressing a nuclearised North Korea is his top foreign policy issue.
Biden said at his first news conference as president that North Korea has violated UN resolutions by launching ballistic missiles and that the United States and its allies will respond if the country escalates the situation.
"I'm also prepared for some form of diplomacy but it has to be conditioned upon the end result of denuclearisation," he said.
He said he agrees with former president Barack Obama's warning that North Korea was the most pressing foreign policy priority to watch.
Four missiles fired this week were all short-range and don't pose a direct threat to the US mainland.
According to the South Korean government's assessment, the first two weapons launched on Sunday were believed to be cruise missiles.
But Japan said the two fired on Thursday were ballistic missiles.
They are more provocative weapons that North Korea is banned from testing by UN resolutions.