North Korea said on Tuesday its main nuclear complex was operating, and it was working to improve the "quality and quantity" of its weapons which it could use against the United States at "any time". The comments followed a declaration by the North in 2013 vowing to restart all nuclear facilities, including the main nuclear reactor in Yongbyon that had been shuttered.

"If the U.S. and other hostile forces persistently seek their reckless hostile policy towards the DPRK [North Korea] and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons any time," the atomic energy director was quoted as saying.

RELATED:  U.S. would show flexibility in nuclear talks with North Korea: envoy

It marked the first acknowledgement that the plant, which has been the source of fissile material used in the country's atomic weapons program, is operational, experts said. The country is believed to be working towards developing an intercontinental ballistic missile mounted with an atomic warhead that can hit targets in the mainland United States.

RELATED: The flying marshal: North Korea builds private runways for plane-loving Kim

Late on Monday, the North's space agency said it was readying a new satellite for launch, indicating it may fire an upgraded long-range ballistic missile timed to the 70th anniversary of its ruling party on Oct.10.

"It has been nearly three years since their last nuclear test and now may be the time for them to test their evolved technology," said Park Jiyoung, an analyst at the Asan Institute for Policy Studies in Seoul.