SINGAPORE — Roller-coaster nuclear diplomacy between the United States and North Korea hit highs and lows on Saturday as the countries’ top diplomats traded polite words and barbs, leaving efforts to rid the North of its atomic weapons at an uncertain juncture.

At a security conference in Singapore, U.S. Secretary of State Mike Pompeo accused North Korea and countries including Russia of continuing to violate U.N. sanctions aimed at pressing Pyongyang to give up its nuclear arsenal. But at the same time, he oversaw the handover of a letter to North Korean leader Kim Jong Un from President Donald Trump and exchanged pleasantries with the North’s top diplomat.

North Korean Foreign Minister Ri Yong Ho, meanwhile, greeted Pompeo with a smile, but then delivered a scathing attack on the Trump administration for approaching the negotiation poorly by insisting on sanctions enforcement. Ri said North Korea would not be forced into acting unilaterally and demanded that the U.S. undertake “confidence building” measures if the negotiation was to be successful.

After Pompeo warned anew that no sanctions would be lifted until North Korea fully and finally denuclearizes, Ri told the annual ASEAN Regional Forum that the North would not be bullied into concessions.

“Confidence is not a sentiment to be cultivated overnight,” he said. “In order to build full confidence between the DPRK and the U.S., it is essential for both sides to take simultaneous actions and phased steps to do what is possible one after another.” North Korea’s official name is the Democratic People’s Republic of Korea.

“Only when the U.S. ensures that we feel comfortable with and come close to it, will we be able to open our minds to the U.S. and show it in action,” Ri said.

The U.S. has previously dismissed calls for a phased approach, insisting that sanctions be maintained until the North delivers on its commitments but suggesting that some other steps may be possible.

Ri, though, appeared unmoved and accused elements of the U.S. government of going against Trump’s wishes by taking a hard line on sanctions.

“What is alarming, however, is the insistent moves manifested within the U.S. to go back to the old, far from its leader’s intention,” he said.

Instead of responding reciprocally to North Korea’s suspension of nuclear tests and missile launches and other goodwill gestures such as the return of suspected remains of American troops killed in the Korean War, the U.S. has maintained hostility, Ri said.