North Korea claims it has successfully performed another "crucial test" at its long-range rocket launch site.
It comes as the country continues to pressure the Trump administration over an end-of-year deadline set by leader Kim Jong Un to salvage faltering nuclear negotiations.
North Korea's Academy of Defence Science did not specify what was tested on Friday but said it would further strengthen its "reliable strategic nuclear deterrent".
Days earlier, North Korea said it conducted a "very important test" at the site.
The announcement sparked speculation that the test involved a new engine for either a space launch vehicle or an intercontinental ballistic missile.
Negotiations broke down after a meeting between US President Donald Trump and Mr Kim in Vietnam in February.
The US rejected the North's demands for relief on sanctions in exchange for a partial surrender its nuclear capabilities.
In June, the two leaders met for a third time at the border between North and South Korea and agreed to resume talks.
But a further meeting in Sweden saw talks grind to a halt again over what the North Koreans described as the Americans' "old stance and attitude".
Following the news of the test, US Ambassador Kelly Craft criticised the North during a UN Security Council meeting, describing it as "deeply counterproductive".
She warned it risked closing the door on further peace negotiations.
Ms Craft also accused North Korea of "a resumption of serious provocations" and said that they could launch space vehicles using long-range ballistic missile technology "which are designed to attack the continental United States with nuclear weapons".
However, the ambassador said the Trump administration is "prepared to be flexible" and work towards resuming talks.
North Korea described Ms Craft's comments as a "hostile provocation" and warned Washington may have squandered its chance of restarting dialogue.
Cover Image: Kim Jong Un laughing after a missile launch earlier this year (Picture: KCNA).