Donald Trump met North Korean Kim Jong-un for the first time in June.
North Korea accused the United States on Thursday of pushing for worldwide sanctions despite goodwill moves by Pyongyang and said that progress on denuclearisation promises could not be expected if Washington continued to follow an "outdated acting script".
The unidentified spokesperson also warned that there is no guarantee that the hard-won atmosphere of stability on the Korean Peninsula will continue.
North Korea has lashed out at the United States for continuing to push for sanctions enforcement.
The White House did not respond to requests for comment. The letter was handed to North Korean Foreign Minister Ri Yong Ho during the weekend.
The country claimed that the U.S.is denying it "basic decorum" and is clinging "to the outdated acting script which the previous administrations have all tried and failed", warning that they "cannot expect any progress in the implementation" of the joint agreement between the two countries reached earlier this year.
The statement accused USA diplomats of "baseless allegations" against North Korea.
In another case, black coal imported from North Korea was transferred to the Russian port of Kholmsk, where it was "disguised as semi-coking coal that does not need country of origin documents", before being reloaded on ships heading to the South, the customs office said.
North Korea appears to continue dismantling a rocket launching site, a USA think tank said, based on satellite images.
Iranian President Hassan Rouhani delivers a statement after a two day visit in Bern, Switzerland, July 3, 2018.
The rival Koreas plan to hold high-level talks on Monday to prepare for a third summit between their leaders, as Pyongyang called on the United States to reciprocate its "goodwill measures" by easing sanctions and stopping demands that the North denuclearize first.
"Rather than obsessing about the score, we will try to serve as a peace delegation with a mission to advance South-North sports exchange", Lee Sung-keun of FC Gangwon said.
They said it had also not agreed to definitions of the key terms of any agreement, or to any inspection of its nuclear test site, which it claims to have decommissioned, but which US intelligence officials have said may still be operable.
But the details of that process remain vague - North Korea did not commit to unilaterally give up its nuclear weapons and the United Nations says its nuclear programme is continuing.
Satellite images from last Friday indicate North Korea has carried out additional activities to dismantle a key missile launch facility, the Washington-based 38 North think-tank said on Tuesday.