China and Russia have broken with the US over the maintenance of sanctions on North Korea, arguing they should be relaxed in view of the “positive development of the past few months”.

The rift became apparent in a session of the UN security council called by the US and chaired by the secretary of state, Mike Pompeo. It suggested that the international consensus over North Korea, which had led to a steady escalation of sanctions since 2006, had stalled in the wake of a flurry of summit diplomacy by Pyongyang, Seoul and Washington.

The security council foreign ministers meeting on Thursday marked the second diplomatic setback for the US in two days, the day after Donald Trump was roundly rebuffed by other world leaders when he sought to gain support for the reimposition of sanctions on Iran.

Pompeo’s task at the security council in maintaining a solidarity over sanctions at a time when the US president has been claiming his summit with Kim Jong-un in Singapore in June represents an historic turning point.

In his own introductory remarks, the secretary of state called it “the first significant diplomatic breakthrough in decades”.

Kim agreed to the “denuclearisation of the Korean peninsula” at the Singapore summit, but the Pyongyang regime interprets the phrase to mean a drawn-out process of multilateral disarmament, not the unilateral dismantling of the North Korean arsenal.

So far it has only begun dismantling a missile engine testing site, which may have become obsolete as North Korean technicians have moved from using liquid fuel to solid fuel for long-range missiles.

In supporting Pompeo’s call for maintaining sanctions, the UK foreign secretary, Jeremy Hunt, gave a more modest assessment of the consequences of Singapore, saying: “We note the lack of concrete steps so far.”

“Action counts for more than assurances,” Hunt said. “Deeds must follow words.”

However, Wang Yi, the foreign minister of China, whose cooperation is essential to enforcing sanctions, said that “given the positive developments” China believed the UN “needs to consider invoking in due course this provision to encourage [North Korea] and other relevant parties to move denuclearisation further ahead”.

His Russian counterpart, Sergey Lavrov, backed the call for loosening the sanctions, arguing: “Any negotiation is a two-way street. Steps by the DPRK toward gradual disarmament should be followed by the easing of sanctions.”