North Korean authorities said on Tuesday the country's main nuclear facility, the Yongbyon complex, is now in "full operation".
The country was improving its nuclear weapons "in quality and quantity", BBC reported citing state-run news agency, KCNA.
The North was ready to face US hostility with "nuclear weapons any time", the report added.
Yongbyon's reactor was shut down in 2007 but Pyongyang vowed to restart it in 2013, following its third nuclear test and amid high regional tensions.
The reactor has been the source of plutonium for North Korea's nuclear weapons programme.
Six-nation talks aimed at ending North Korea's nuclear programme have been stalled since early 2009.