The alarming activity was detected by US spy planes, and indicated the barbaric regime is continuing to pursue its missile programme.
Pyongyang has loaded two Stormpetrel anti-ship cruise missiles onto a Wonsan guided-missile patrol boat at Toejo Dong, on the country’s east coast.
It is the first time since 2014 these types of weapons have been deployed to ships.
The aggressive action signals despotic leader Kim Jong-un is ignoring threats and sanctions from the West, at whatever cost.
Secretary of State Rex Tillerson said: “The best signal that North Korea could give us that they're prepared to talk would be to stop these missile launches.”
The regime has conducted multiple missile launches - with both successes and failures - over the past 18 months.
Kim Jong-un has flagrantly ignored escalating sanctions slapped on it by the UN and the West, instead issuing increasing threats against Donald Trump and the USA.
The US has hit back testing its own anti-ICBM capabilities.
The Pentagon successfully shot down an ICBM - the same type of missiles North Korea is perfecting - in a public show of strength in May.
A US official, speaking on condition of anonymity, said: “North Korea is not showing any evidence it plans to halt its missile tests.
“It's a trend that does not bode well for hopes of de-escalating tensions on the [Korean] peninsula.”
The latest action indicates another missile launch could be imminent, or could be pre-empting actions by the US.
The US Navy has already dispatched warships to the Korean peninsula, and Pyongyang may fear more are on their way.
Further action is being taken by the international community to curb North Korea’s nuclear desires, with more sanctions being voted through unanimously by the UN Security Council.
But Mr Trump voiced his anger over the coverage awarded to the sanctions, tweeting: “The Fake News Media will not talk about the importance of the United Nations Security Council's 15-0 vote in favor of sanctions on N. Korea!"
The US plans to bolster its defences in the region faced delays after the incoming South Korean administration put the brakes on deploying the Terminal High Altitude Area Defense (THAAD) missile defence system in the country.
China, the powerhouse in the region, has repeatedly voiced its opposition to the missile system.
The plans are back on track for the time being, with Seoul agreeing to temporary deployment of THAAD dependent on the outcome of an environmental survey.
Talks between South Korean and the US officials could see changes to the policy over rules governing the size of warheads South Korea is allowed to mount on its missiles.
Captain Jeff Davis, a Pentagon spokesman, said: “Yes, we are working on it.
"It's a topic under active consideration here, and I would tell you that we would be favorably inclined to do anything which furthers the defensive capabilities of South Korea."