South Korea and the USA, meanwhile, have launched a massive military exercise involving some 12,000 military personnel and approximately 250 aircraft, including bombers and stealth fighters.
A senior United Nations official has met with North Korea's vice foreign minister on the first full day of a rare trip to Pyongyang.
His visit comes amid high tensions on the Korean Peninsula following North Korea's intercontinental ballistic missile launch last Wednesday.
"The large-scale nuclear war exercises conducted by the U.S.in succession are creating [a] touch-and-go situation on the Korean peninsula and [a] series of violent war remarks coming from the US high-level politicians amid such circumstances have made an outbreak of war on the Korean peninsula an established fact", the state-run Korean Central News Agency quoted the official as saying.
"[It's no longer a question of] if war breaks out, but a matter of when".
The US and South Korea kicked off the five-day war games - called Vigilant Ace - on Monday, with more than 200 aircraft, which have been conducting mock attacks on North Korea in different war scenarios. It also labeled US President Donald Trump as "insane".More news: Australian Parliament allows same-sex marriage
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Midway though the large-scale aerial exercises involving hundreds of warplanes, the United States has flown a B-1B supersonic bomber over South Korea.
North Korean leader Kim Jong-un visits the Amnokgang Tire Factory in Chagang Province, North Korea on December 3, 2017.
B-1Bs flyovers have become an increasingly familiar show of force to North Korea, which after three intercontinental ballistic missile tests has clearly moved closer toward building a nuclear arsenal that could viably target the USA mainland.
McMaster's remarks even saw Republican Senator Lindsey Graham say it was necessary to move all the United States military dependents out of the peninsula for fear of war breaking out.
Six U.N. agencies, with about 50 global staff, are represented in the North.
North Korea is not interested in escalation of tensions on the Korean Peninsula, but is not afraid of war, Chairman the Federation Council Committee for Foreign Affairs Konstantin Kosachev said after a meeting with North Korea's Ambassador to Russia Kim Hyong-jun. According to political experts, China has conducted the drills intentionally to send a message to the US.