North Korea has denounced US calls for international sanctions to be enforced despite Pyongyang’s goodwill moves, saying progress on denuclearisation cannot be expected if Washington follows an “outdated acting script”.

The spat comes as South Korea’s customs office said three companies had imported North Korean coal and iron from Russia by forging documents in an apparent violation of United Nations sanctions.

North Korea’s foreign ministry said on Thursday that Pyongyang had stopped nuclear and missile tests, dismantled a nuclear test ground and returned the remains of US soldiers killed in the 1950-53 Korean war. Yet Washington was still insisting on “denuclearisation first” and had “responded to our expectation by inciting international sanctions and pressure”.

“As long as the US denies even the basic decorum for its dialogue partner and clings to the outdated acting script which the previous administrations have all tried and failed, one cannot expect any progress in the implementation of the DPRK-US joint statement including the denuclearisation,” the ministry said.

The statement on the KCNA state news agency said North Korea, which calls itself the Democratic People’s Republic of Korea, was still willing to implement a broad agreement made at a landmark 12 June summit between Donald Trump and Kim Jong-un.

But it accused unidentified high-level US officials of “going against the intention of President Trump” by “making baseless allegations against us and making desperate attempts at intensifying the international sanctions and pressure”.

It said “expecting any result, while insulting the dialogue partner” was “a foolish act that amounts to waiting to see a boiled egg hatch out”.

North Korea’s foreign minister, Ri Yong-ho, demanded the US “make good on its part of the promise” and declared: “Since we know that the US will never give up its hostile policy toward us, we will hold on to our nuclear knowledge.”

Repeated calls in North Korea’s state-run media this week have urged the US to drop sanctions. But there have also been well documented violations of those restrictions, with the latest in the South.

More than 35,000 tons of coal and pig iron worth 6.6bn won (£4.6m) were imported into South Korea between April and October last year, South Korean authorities said. The shipments were first sent to a Russian port before arriving in the South and the three firms complicit in the deception are being prosecuted. The illicit cargo may have violated UN sanctions designed to punish the North for its nuclear programme, and arrived in South Korea during the height of tensions last year.

The White House did not respond to requests for comment.

Kim vowed in Singapore to work towards denuclearisation but the two sides have yet to define a deal to meet that goal and Washington insists sanctions pressure must be maintained during negotiations.

The North Korean statement followed comments this week by top American diplomats stressing the need for Pyongyang to take additional steps toward denuclearisation.

Trump’s national security adviser, John Bolton, said this week that Pyongyang had not taken the necessary steps to denuclearise while US ambassador to the United Nations Nikki Haley said Washington was “not willing to wait for too long”.

Bolton, who spoke on Tuesday, said Trump, in a letter to Kim, proposed sending Pompeo back to North Korea, and that Trump was ready to meet with Kim any time.

State department spokeswoman Heather Nauert said on Thursday that the United States was in touch with North Korea virtually every day or other day, but had no travel plans to announce for the moment.

US officials have declined to comment on a report on the Vox news website on Wednesday saying that North Korea repeatedly rejected a US proposal for it to cut its nuclear arsenal by 60-70% within six to eight months.

US officials familiar with the talks, however, told Reuters that North Korea had yet to agree to a timeline for eliminating its nuclear arsenal or to disclose its size, which US estimates have put at between 30 and 60 warheads.

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.