The US secretary of state came face-to-face with North Korea's ambassador to the UN for the first time in a decade on Friday, as Rex Tillerson confronted Ja Song Nam at a Security Council meeting in New York.
Mr Tillerson offered an olive branch to the North Koreans, telling them that "all options remain on the table" and urging them to abandon their nuclear programme and return to negotiation.
"We do not seek, nor do we want, war with North Korea," he said.
"The United States will use all necessary measures to defend itself against North Korean aggression, but our hope remains that diplomacy will produce a resolution."
He said that "a sustained cessation of North Korea's threatening behaviour must occur before talks can begin," and added: "North Korea must earn its way back to the table. The pressure campaign must, and will, continue until denuclearisation is achieved.
"We will, in the meantime, keep our channels of communication open."
He also issued a stern rebuke to Russia and China, accusing them of supporting Pyongyang.
"Continuing to allow North Korean labourers to toil in slave-like conditions inside Russia in exchange for wages used to fund nuclear weapons programmes calls into question Russia's dedication as a partner for peace," he said.
"Similarly, as Chinese crude oil flows to North Korean refineries, the United States questions China's commitment to solving an issue that has serious implications for the security of its own citizens."
Mr Ja, an elusive figure in New York, responded with a rebuke to the US, saying that they were trying to bully the UN to turn against his country.
He said his country would not pose a threat to any state, as long as its interests were not infringed upon.
And he described Mr Tillerson's speech as "a desperate measure plotted by the US being terrified by the incredible might of our Republic that has successfully achieved the great historic cause of completing the state nuclear force."
Speaking after the session, Mr Tillerson did not reiterate his comments earlier this week that there were no preconditions to talks.
Instead, he emphasised that North Korea must end its provocative behaviour before talks begin.
"Militarily we are going to be prepared, should something go wrong," he warned.
His comments came the day after President Donald Trump talked to Vladimir Putin, for the third time in six weeks. The US president said the call was primarily about getting Russia's co-operation on North Korea but added that Mr Putin made some "very nice" comments about his time in office.