The United States has warned North Korea of consequences if it makes good on threats of a spectacular weapons test in the new year, while offering flexibility if it stayed in talks.

Frustrated by the lack of sanctions relief after three summits with President Donald Trump, North Korea has vowed an ominous “Christmas gift” if the US does not come up with concessions by the end of the year.

At the UN security council, the US ambassador, Kelly Craft, voiced concern that North Korea was indicating it would test intercontinental ballistic missiles “which are designed to attack the continental United States with nuclear weapons”.

“Missile and nuclear testing will not bring the DPRK greater security,” Craft said, referring to the North by its official name, the Democratic People’s Republic of Korea. “We trust that the DPRK will turn away from further hostility and threats, and instead make a bold decision to engage with us,” she said.

Hinting at further sanctions, Craft said: “If events prove otherwise, we, this security council, must all be prepared to act accordingly.”

She called North Korea’s persistent shorter-range tests “deeply counterproductive” and, in an unusually direct condemnation by the US, said they violated UN security council resolutions.

Craft appeared to rule out meeting North Korea’s demands for an offer in the year’s final weeks, saying: “Let me be clear: The United States and the security council have a goal – not a deadline.”

But she said the US, which used its presidency of the council to convene the meeting, wanted to work toward a deal. “We remain ready to take actions in parallel, and to simultaneously take concrete steps towards this agreement,” Craft said. “We are prepared to be flexible in how we approach this matter.”

Trump has hailed his relationship with North Korean leader Kim Jong-un after his breakthrough summit last year in Singapore and two subsequent meetings.

Trump, who has boasted that he prevented war, has said he would be “surprised” if North Korea acted in a hostile manner.

But North Korea – which did not participate in Wednesday’s session – has shown frustration since February’s eagerly awaited summit in Hanoi ended in stalemate.

The two countries met in Stockholm in October to start working-level talks, though North Korea denounced the meeting, accusing the US of not offering anything.

China and Russia, which had cautiously backed pressure against North Korea after its past nuclear tests, indicated they would reject further sanctions.

“The security council must revisit sanctions to alleviate the suffering of the North Korean people and create a fruitful environment for dialogue,” said ambassador Zhang Jun of China, North Korea’s primary ally.

Russia’s UN ambassador, Vassily Nebenzia, faulted US diplomacy, saying: “You can’t reach an agreement if you don’t propose anything in exchange.”

But Japan and South Korea both put the onus on North Korea. South Korea – whose dovish president, Moon Jae-in, paved the way for Trump’s outreach to Pyongyang – urged its neighbour to abide by council resolutions.

North Korea should “remain meaningfully engaged in dialogue with the United States and [South] Korea so that progress in negotiations can lead to concrete results,” said its ambassador, Cho Hyun.

France and Britain, which both wield veto power on the security council, also called on North Korea to seize the rare opportunity offered by Trump for diplomacy.

“It is not too late,” Karen Pierce, the British ambassador, said to North Korea. “You can stop this situation from worsening.”