His comments were made to reporters in the Chinese city of Xiamen after the annual BRICS (Brazil, Russia, India, China and South Africa) summit of five emerging economies.
They came after North Korea detonated what it called a hydrogen bomb on Sunday and announced it could mount the warhead on a missile, dramatically raising the stakes in Pyongyang's standoff with the international community over its banned weapons programs.
North Korea has been spotted moving what appears to be an intercontinental ballistic missile towards its west coast, according to reports.
The rocket started moving on Monday, a day after the North's sixth nuclear test, an unidentified intelligence source has now revealed to South Korea's Asia Business Daily.
It was spotted moving under cover of darkness to avoid surveillance, the report said. North Korea has launch facilities for its missile program on its west coast.
The destination could be Sohae Satellite Launching Station, a major intercontinental ballistic missile development and testing site around 200 kilometres northwest of Pyongyang, or a missile silo in mountainous Geumchang-ri, North Pyongan province.
But Brendan Thomas-Noone from the US Studies Centre said North Korea now has technology that allows it to launch missiles from road mobile launchers, on any flat surface.
It could also fire a rocket over its own land, meaning the missile could head in any direction.
'They'll keep pushing until they can strike the US'
Thomas-Noone told news.com.au Kim Jong-un could be purposely moving the rocket to demonstrate that the United States and South Korea will struggle to track and destroy its weapons.
"It came from one of the missile factories on the west coast known to produce ICBMs," he said.
"The last couple of tests have shown they can launch from multiple locations.
"They are obviously saying they're working to keep pushing the envelope until they are confident they can strike the US with an ICBM."
North Korea has already shown its ICBMs could threaten the US mainland by testing two in July that were capable of flying 10,000 kilometres.
"The last missile test fired directly up into the atmosphere - they can test missile distance without firing over land - this one started because they want to test the distance over land," said Mr Thomas-Noone. "I expect them to do that."
South Korea's defence ministry said they were unable to confirm the report that North Korea is moving a rocket, but the ministry said in parliament on Monday that the North was considered ready to launch more missiles, including ICBMs, at any time.
It comes amid speculation the rogue nation may be planning to fire an ICBM as early as this weekend, when the republic celebrates its foundation on September 9.
South Korea sends strong warning
South Korean warships conducted further live-fire exercises at sea on Tuesday as Seoul continued its displays of military capability after North Korea's most powerful nuclear test to date.
South Korea's presidential office said Washington had agreed to remove bilaterally agreed warhead restrictions on Seoul's missiles, to allow it to develop bigger weapons that would boost its pre-emptive strike capabilities against the North.
The South's military exercises on Monday involved F-15 fighter jets and land-based ballistic missiles simulating an attack on North Korea's nuclear test site to "strongly warn" Pyongyang over the recent detonation.
The North's underground test of what it claimed was a hydrogen bomb on Sunday had an estimated strength of 50 kilotons, according to the acting chief of South Korea's defence ministry.
North Korea released photos of Kim next to what it said was a bomb designed for an ICBM.
Pyongyang called the device a "thermonuclear weapon with super explosive power" entirely made "by our own efforts and technology."
South Korea's military on Monday responded to North Korea's nuclear test with live-fire exercises off its eastern coast involving ground and air-launched rockets.
The South fired short-range Hyunmoo missiles into the sea to simulate an attack on the North's main nuclear test site.
