Despite the agreement to denuclearise the Korean peninsula struck during a landmark summit between North Korean leader Kim Jong-Un and US President Donald Trump, "we preserve our nuclear science as we know that the Americans will not abandon their hostility toward us," Ri said, according to the conservative Mehr news agency.

"Dealing with Americans is difficult, and as our main goal is total disarmament of the whole Korean Peninsula, it is necessary that the Americans also abide by their commitments but they refuse to do so."

At the June summit with Trump, Kim made a vague commitment to denuclearisation -- far from the longstanding US demand for the complete, verifiable and irreversible dismantling of Pyongyang's atomic arsenal.

Ri was meeting with Iran's influential parliament speaker Ali Larijani on the third day of an official visit.

"The Americans utter beautiful words when negotiating and promise a very bright future but they deliver on none of their commitments when it comes to action," Larijani said.

Ri arrived in Tehran on the same day as the United States reimposed sanctions after abandoning a 2015 nuclear deal between major powers and Iran.

The other five parties to the deal agree with UN inspectors that Iran has been abiding by its commitments, but Trump has said repeatedly it is "a horrible deal" and announced he was abrogating it in May.