The dictatorship has attracted widespread condemnation after refusing to step back from aspirations of becoming a fully-fledged nuclear power, despite strong warnings from members of the UN Security Council.
The confidential analysis was completed last month by the US Defense Intelligence Agency, reports The Washington Post. The NBC source did not have that level of specificity.
The white paper stated that North Korea has reached a "new stage" in its weapons program following successful tests of its intercontinental ballistic missiles in recent weeks. "The North Koreans have a plan to use nuclear weapons very early in a conflict".
According to the U.S. military's recent declaration, the United States has 806 deployed ICBMs, SLBMs (submarine-launched ballistic missile), and heavy bombers as well as 1,722 deployed nuclear warheads.
The report comes after North Korea launched an improved ballistic missile with intercontinental range in July, Pyongyang's second missile launch in less than a month.
The North's official news agency said it was mulling a plan to fire medium-to-long-range rockets around Guam, where United States strategic bombers are based.
It comes as tensions between North Korea and the U.S. continue to increase, with Kim Jong-un threatening "thousands-fold" revenge against the USA after the United Nations passed a new set of stringent sanctions against the one-party state.
North Korea on Monday rejected the United Nations sanctions over its long-range missile tests and vowed to make Washington pay a price "dearly" with its retaliation.
Separately, Japan's defense ministry assessed in a report Tuesday that it is possible that North Korea has achieved the miniaturization of nuclear weapons and has developed nuclear warheads.
Turei resigns as co-leader
Earlier this week, Green MPs Kennedy Graham and David Clendon resigned in protest at Ms Turei's actions. Labour is changing its signs which now have Ms Ardern and former leader Andrew Little on them.
The United Nations Security Council unanimously approved new sanctions-with the rare support of Russian Federation and China-to ban coal and other exports to North Korea valued at more than $1 billion.
"The nation's state-run news agency responded that "(North Korea) will make the US pay dearly for all the heinous crimes it commits against the state and people of this country".
Analysts have long suspected North Korea could make a small-enough device to launch atop a missile, but this is the first known internal government document to acknowledge the technology.
"North Korea had best not make any more threats to the United States".
Thousands of U.S. soldiers are stationed in South Korea and Japan.
North Korea claimed to have tested the smaller design a year ago, but that has not been confirmed.
Technically, the US and North Korea are still in a state of war.
Republican US Senator John McCain said that Trump should tread extremely cautiously when issuing threats to North Korea unless he is prepared to act. The resolution approved on Saturday bans North Korean exports of coal, iron, iron ore, lead, lead ore and seafood - resources that are worth more than $1 billion to the regime.