The United States warned Wednesday that North Korea will face "severe consequences" if it continues with what Washington sees as its provocative decision to restart a nuclear reactor.

Pyongyang has said it has restarted the long-mothballed Yongbyon reactor, which is capable of producing weapons-grade plutonium, and has threatened to launch a rocket which US officials see as a test-bed for ballistic missile technology.

Challenged as to whether Washington could respond credibly to the North Korean moves after earlier striking a deal to allow Iran's nuclear program to continue under international supervision, Secretary of State John Kerry insisted it could.

"There will be severe consequences as we go forward if North Korea does not refrain from its irresponsible provocations that aggravate regional concerns, make the region less safe, and if it refuses to live up to its international obligations," he said.

"Our position is clear: We will not accept a DPRK -- North Korea -- as a nuclear weapons state, just as we said that about Iran."

Asked what the United States could do if North Korea continues to flout international agreements not to expand its nuclear and missile programs, Kerry said Kim Jong-Un's regime was already experiencing growing diplomatic isolation.

"China, for instance, has taken serious steps in the last year, year-and-a-half, since we engaged China on this subject specifically to encourage them to do more, and they have," he told reporters at the State Department.

Kerry said he had also spoken to his Russian counterpart Sergei Lavrov about North Korean defiance.

"So there's a lot happening. And I can assure you that all of these countries remain fixated on the need for North Korea to denuclearize with respect to its weapons program and to live up to its international obligations," he said.

North Korea mothballed the Yongbyon reactor in 2007 under a six-nation aid-for-disarmament accord but began renovating it after its latest nuclear test in 2013.

When fully operational the reactor is capable of producing around six kilograms (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.