In a statement carried by the North's state-run Korean Central News Agency on Tuesday, an atomic energy official said Pyongyang is improving its nuclear weapons arsenal "in quality and quantity."
"If the U.S. and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons any time," the director of the North Korean Atomic Energy Institute said, using an abbreviation of the country's official name, the Democratic People's Republic of Korea.
North Korea's main nuclear complex at Yongbyon, which includes a uranium enrichment plant and a plutonium production reactor, is operating normally, the official told the news agency.
Notorious for issuing alarming and attention-grabbing statements, Pyongyang has repeatedly threatened to use nuclear weapons against the United States. But strong doubts remain
over whether it has the missile technology to target the U.S. mainland.
In an indication it wants to advance its missile capabilities, North Korea said Monday
it was planning more satellite launches. Prohibited by U.N. Security Council resolutions, such launches are widely seen as a way of testing ballistic missile technology.
Kim's regime didn't say when the next launch would take place, but observers have speculated that it could launch a long-range rocket carrying a satellite in October around the 70th anniversary of North Korea's ruling party.
The atomic energy official on Tuesday reiterated the North Korean stance that its nuclear weapons program is a self-defense measure "in the face of the U.S. extreme hostile policy and nuclear threats towards it."
U.S. stresses missile defense measures
In an interview with CNN earlier Tuesday, U.S. Ambassador to South Korea Mark Lippert said Washington is constantly working with Seoul "to ensure that other allies in the region as well as the U.S. homeland are protected from threats posed by North Korea."
"We've moved, over time, a good deal of missile defense capability to the region," Lippert said before North Korea issued the statement about its nuclear program. "Ground-based interceptors to Alaska, surface combatants to the Western Pacific, a THAAD battery on Guam, another radar in Japan in order to be ready and vigilant for anything the North Koreans may or may not do."
THAAD, which stands for Terminal High Altitude Area Defense, is a ballistic missile defense system.
After the North Korean statement, U.S. State Department spokesman John Kirby told a regular news briefing that the United States continues "to call on North Korea to refrain from irresponsible provocations that aggravate regional tensions, and instead focus on fulfilling its international obligations and commitments."
Nuclear complex revamped
The North's announcement that the Yongbyon nuclear complex is up and running comes as little surprise.
During a period of heightened tensions in the region in spring 2013, Pyongyang announced it would revamp and restart the facilities at the site.
In February of this year, U.S. Director of National Intelligence James Clapper said he believed that "North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the reactor."
But some North Korea watchers have questioned whether the reactor is operating at full power.
A July article on 38 North
, a website that specializes in analysis of North Korea, said satellite imagery suggested the reactor may not have been operating or was only functioning at low power levels.
The report's authors also identified rapid construction at the uranium enrichment plant of a building they theorized could be used "to assemble or store conventional high explosive components of a nuclear weapon."
The North Korean atomic energy official said Tuesday that the Yongbyon facilities were being employed for both economic development and "the building of a nuclear force."
Expert says nuclear arsenal is growing
Kim Jong Un's regime may already have 10 to 15 nuclear weapons, according to David Albright, a former U.N. weapons inspector who now heads the nonprofit Institute for Science and International Security.
In a report in February, Albright predicted that Pyongyang could increase its stockpile to anywhere between 20 and roughly 100 nuclear weapons by 2020
The growing nuclear arsenal poses a serious strategic challenge for the United States.
The U.S. government has repeatedly called on North Korea to commit to denuclearization as a condition of any future negotiations, but Kim's regime has repeatedly dismissed such an idea, demanding to be recognized as a nuclear power.
Pyongyang's statement Tuesday provided few details about its specific grievances with U.S. policy, which it accused of "openly seeking the downfall" of North Korea's "social system."
The combative rhetoric comes just three weeks after North and South Korea reached a deal
to dial down tensions in the region that were inflamed by landmine blasts and artillery fire in the Demilitarized Zone that separates the two countries.
South Korean President Park Geun-hye issued a joint statement with EU leaders after a summit in Seoul on Tuesday in which they strongly condemned the North's "continued development of its nuclear and ballistic missile programs."
The leaders urged North Korea to abandon all its nuclear weapons and existing nuclear program in a "complete, verifiable, and irreversible manner," and to "refrain from any further provocation."