Speaking Wednesday at a joint press conference with South African Foreign Minister Maite Nkoana-Mashabane, Kerry said the imposition of sanctions against Iran, which also has aspirations as a nuclear power, influenced Tehran to change its direction and end their isolation from the international community.

But he said Pyongyang's "almost total absence of a legitimate economy" meant that the same strategy may not work with North Korea.

Kerry warned there will be "severe consequences" if North Korea does not refrain from "its irresponsible provocations that aggravate regional concerns, make the region less safe, and refuse -- if it refuses to live up to its international obligations. Our position is clear: We will not accept a DPRK -- North Korea -- as a nuclear weapons state, just as we said that about Iran."

Ready to use nuclear weapons

Kerry was speaking the day after North Korea warned it is ready to use nuclear weapons against the United States and other foes if they pursue "their reckless hostile policy" toward it.

READ: Defector: Kim's days are numbered

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.

Attention-grabbing

Notorious for issuing alarming and attention-grabbing statements, the isolated regime 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.

READ: Carter: North Korea has no chance of U.S. victory

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 Jong Un'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.

CNN's Rebecca Wright contributed to this report.