Despite "goodwill measures" taken by the North, Washington was "raising its voice louder for maintaining the sanctions against the DPRK," the North's Foreign Minister Ri Yong Ho told a security forum in Singapore, according to a statement. DPRK are the initials of the North's official name.

He criticised "insistent moves manifested within the US to go back to the old, far from its leader's intention".

"Impatience is not helpful at all for building confidence. Especially, advancing unilateral demands will further deepen mistrust, instead of reviving trust," said the statement.

"As long as the US does not show in practice its strong will to remove our concerns, there will be no case whereby we will move forward first unilaterally," Ri added.

Earlier Pompeo urged major powers attending the same forum, including China and Russia, to keep up sanctions pressure against the North.

At landmark talks in June with US President Donald Trump, the North's leader Kim Jong Un signed up to a vague commitment to "denuclearisation of the Korean Peninsula" -- a far cry from long-standing US demands for complete, verifiable and irreversible disarmament.

US officials have publicly been optimistic about the agreement but progress appears to have been slow.