The United States said Thursday that any deal reached with Iran to curb its nuclear ambitions would be "fundamentally different" from a pact sealed with North Korea that later unraveled.

"The restrictions, inspections and verifications measures imposed on Iran by a comprehensive plan of action will go far beyond those placed on North Korea in the 1990s and the 2000s," said acting State Department spokeswoman Marie Harf.

She was quizzed about reports that Chinese nuclear experts believe North Korea may already have a nuclear arsenal of 20 warheads and the uranium enrichment capacity to double that figure by next year.

The estimate, which The Wall Street Journal said was relayed to US nuclear specialists in a closed-door meeting in February, is significantly higher than any previously known Chinese assessment.

It also exceeds recent estimates by independent US experts which put the North's current arsenal at between 10 and 16 nuclear weapons.

Harf refused to discuss what the US administration estimates Pyongyang's current stockpile of nuclear weapons to be.

But she refuted allegations from critics of the current Iran nuclear negotiations that the situation in North Korea should raise concern.

"The comprehensive deal we are seeking to negotiate with Iran is fundamentally different than what we did in terms of our approach to North Korea," Harf told reporters.

"In the early 1990s, North Korea had produced weapons-grade plutonium prior to agreeing to limited IAEA inspections. After the agreed framework, they agreed to more intrusive inspections; but in 2002, when they finally broke its commitments, its violations were detected by the IAEA."

And she acknowledged that part of the reason for the in-depth, complex technical annexes to an Iran deal was "because of the lessons we learned from the North Korea situation."

International six-party talks aimed at reining in Pyongyang's nuclear program collapsed in 2008.

North Korea carried out nuclear tests in 2006, 2009 and 2013, and has an active ballistic missile development program.