Barack Obama administration advised North Korea to avoid “irresponsible provocation” after North Korea said on Tuesday that its main nuclear complex began operation and it was working to improve the "quality and quantity" of its weapons which could be used against the United States at "any time."
The US added that the North should "focus instead on fulfilling its international obligations."
White House spokesman Josh Earnest stated that the international community would not approve North Korea as a nuclear state.
"We will work with our partners in the context of the six-party talks to try to return North Korea to a posture of fulfilling those commitments that they have made," Earnest said.
South Korea, the US, China, Japan and Russia have undergone numerous sessions of negotiations aimed at ending North's nuclear programme but have been stalled since 2009.
The reactor at Yongbyon Nuclear Scientific Research Center, which is believed to have been the source of plutonium for North's nuclear weapons programme, has been shut down in 2007 as part of a disarmament-for-aid deal.
However - in 2013 - Pyongyang promised to resume the programme, which experts claim that when used at its full capacity, the Yongbyon reactor can make one nuclear bomb's worth of plutonium per year.
Satellite images suggested work had already begun at the complex, according to a statement made earlier this year by a US think-tank.
Tuesday’s announcement proves the first official affirmation that North Korea had restarted operations there.
The state-run news agency KCNA said in a statement that Pyongyang was working on enhancing its nuclear weapons "in quality and quantity," adding that the North was ready to face US animosity with "nuclear weapons any time."
However, experts argue that North Korea's nuclear capabilities are still unclear.
Pyongyang claims it has created a device small enough to fit a nuclear warhead on to a missile, which it could use to launch at its foes.
However, US officials have doubted such claims made by North Korea.
North Korea has been making continuous threats against its neighbours and the US, which often coincide with the annual joint military exercises held by South Korea and US forces.