North Korea on Friday launched a scathing attack on US Secretary of State Mike Pompeo, calling him a "diehard toxin" and saying it was "sceptical" whether it can negotiate with him.
Pyongyang also vowed to remain the United States's "biggest 'threat'" and said annual US-South Korea joint military drills had "complicated" nuclear talks between the two countries.
"We are ready for both dialogue and stand-off," North Korea's foreign minister Ri Yong Ho said in a statement.
"The US is sadly mistaken if it still thinks of standing in confrontation with the DPRK with sanctions, not dropping its confrontational stand," the statement said.
"We will remain as the biggest 'threat' to the US for long and long and will make it understand for sure what it has to do for the denuclearisation."
North Korea carried out several short-range missile tests in recent weeks in protest against the US-South Korean military exercises, which it sees as a rehearsal for invasion.
The launches, and the statement, worsen prospects for a revival of talks between Pyongyang and Washington, which have stalled since a second summit between US President Donald Trump and North Korean leader Kim Jong Un in Hanoi in February collapsed without an agreement.
The pair met again in June in the Demilitarised Zone dividing North and South Korea, and agreed there to restart working-level dialogue, but those talks have yet to begin.
Earlier this week, Stephen Biegun, the US special envoy for North Korea, said during a visit to South Korea's capital that the US was "prepared to engage" as soon as it hears from Pyongyang.
Pyongyang's attack on Pompeo came after he told The Washington Examiner newspaper that if North Korea's leader doesn't denuclearise, "we'll continue to keep on the sanctions that are the toughest in all of history".
The North's foreign minister countered: "He who has no shame has no conscience."
Of Pompeo, he said: "He is truly impudent enough to utter such thoughtless words which only leave us disappointed and sceptical as to whether we can solve any problem with such a guy."
Earlier this month, Trump tweeted that Kim had sent him a letter -- hand-delivered to him -- in which he expressed hope that talks would resume once the joint exercises are over.
Washington stations nearly 30,000 troops in the South to help defend it from its neighbour.