Japan and South Korea have both held joint military exercises with the US in response to a missile launch by North Korea.
On Tuesday, nuclear-armed North Korea test-fired a ballistic missile further than ever before - sending it soaring over Japan for the first time in five years.
Speaking with AM on Wednesday, nuclear nonproliferation expert George Moore said it wasn't yet clear whether a nuclear weapon could be fired off the missile - which was believed to be an intermediate-range ballistic missile.
"Certainly it has the capability - if developed properly - to take a nuclear warhead," said Dr Moore, a former senior analyst at the International Atomic Agency's nuclear security office.
Dr Moore said North Korea had an advanced nuclear programme.
"They potentially have the capability of putting that [nuclear warhead] on a missile," he told AM host Ryan Bridge. "What stage they're in putting it on a missile and what stage they're in finally weaponising a device is not totally clear at this time."
North Korea faced increasing international criticism over its missile testing. Tuesday's test forced the Japanese government to order citizens to seek shelter.
The test was "a reckless act and I strongly condemn it", Japan Prime Minister Fumio Kishida said.
In a statement, the White House said US President Joe Biden and Kishida confirmed in a phone call "they would continue to closely coordinate their immediate and longer-term response bilaterally, trilaterally with the Republic of Korea, and with the international community".
"They resolved to continue every effort to limit the DPRK's (North Korea's) ability to support its unlawful ballistic missile and weapons of mass destruction programs," the White House said.
The move from North Korea came amid rising fears Russia was preparing to use nuclear force in Ukraine. Britain's The Times newspaper reported Russian President Vladimir Putin was currently planning a nuclear test on the Ukrainian border.
But Dr Moore doubted Putin would "test anything on the border".
"You can risk spreading contamination into the Russian Federation or into adjoining areas," he said.
"I think it might be more likely he would test at his own test site to show people he's really serious, or he would pick a rather unpopulated area - more or less in western Ukraine."
Putin had multiple missiles that could carry a nuclear warhead, Dr Moore said.
