HR McMaster says "there's not much time left" to address the issue without armed conflict
President Trump's national security adviser says the possibility of war with North Korea is increasing every day.
HR McMaster says the US is "in a race" to address the nuclear threat from Kim Jong-Un's regime.
Speaking at a forum in California on Saturday, Mr McMaster said: "I think [the risk of war is] increasing every day, which means that we are in a race to be able to solve this problem.
"There are ways to address this problem short of armed conflict, but it is a race because he's getting closer and closer, and there's not much time left."
It comes as North Korea warned that a "reckless" joint military drill by the US and South Korea could spark a nuclear war.
The two allies are set to begin their largest ever co-operative air exercise - dubbed Vigilant Ace - on Monday, with 230 aircraft, including F-22 Raptor stealth jet fighters, ready to take flight.
It is scheduled to last five days and comes less than a week after the rogue state's latest intercontinental ballistic missile test, which Pyongyang claimed was capable of hitting the US mainland.
The Hwasong-15 missile is believed to have reached the highest altitude ever recorded by a North Korean missile.
It flew 950 km before splashing down in waters near Japan, with the secretive state claiming it is capable of striking the entire US mainland.
In an editorial on Sunday, North Korea's ruling party Rodong newspaper said the US-South Korea drill was "an open, all-out provocation" and that it "may lead to a nuclear war at any moment".
The piece - which came shortly after leader Kim Jong Un visited a military tyre factory - added: "The US and South Korean puppet warmongers would be well advised to bear in mind that their DPRK-targeted military drill will be as foolish as an act precipitating their self-destruction."
Mr Kim's factory trip saw him thank workers who had built the tyres for the huge vehicle used to transport the tested Hwasong-15 missile this week, which had prompted a public rally and fireworks in Pyongyang as the country continued to flout international sanctions.
North Korea has been eligible for even harsher sanctions since US President Donald Trump declared it a designated state sponsor of terrorism last month.