The US envoy on the North Korean nuclear issue said Monday that Washington's recent deal with Tehran amply demonstrated its openness in denuclearisation negotiations.

The Iran agreement is "an excellent example of US flexibility and willingness to engage with countries with whom we have had longstanding differences," Sydney Seiler told reporters after meeting his South Korean counterpart in Seoul.

It also shows that the "door is open" to North Korea should it choose to break away from its diplomatic and economic isolation, Seiler said.

Seiler is the US envoy to the six-party talks -- a forum aimed at ending North Korea's nuclear weapons program through negotiations.

The six parties involved -- the two Koreas, China, Japan, Russia and the United States -- have not met for more than six years, and every effort to revive the dialogue process has stalled.

"The Iran deal demonstrated the value and possibilities that negotiations bring," Seiler said, calling on Pyongyang to "choose a different path".

His comments came a week after North Korea stated it had no interest in following Iran down the path of nuclear dialogue and argued that -- unlike Iran -- it was already a nuclear-armed state.

Both Tehran and Pyongyang, allies since Iran's 1979 Islamic revolution, have been subjected to tough economic sanctions over their controversial nuclear programmes.

The deal reached with Iran was touted by some as a possible blueprint for eventual negotiations with North Korea, with US Undersecretary of State Wendy Sherman saying she hoped it would give Pyongyang "second thoughts" about the nuclear path it was pursuing.

But the North's foreign ministry spokesman said last week that the two situations were "quite different".

North Korea "is a nuclear weapons state both in name and reality and it has interests as a nuclear weapons state," he said.

North Korea has staged three successful nuclear tests in 2006, 2009 and 2013.

A recent report by US researchers warned that North Korea was poised to expand its nuclear programme over the next five years and, in a worst-case scenario, could possess 100 atomic weapons by 2020.