The diplomat, speaking on condition of anonymity because consultations have been private, said the North's claim that it tested a hydrogen bomb represents "a step change" and the council needs to show that its response also represents "a step change," even if the claim proves to be untrue, which is likely.
While there is widespread skepticism over the H-bomb claim, whatever the North detonated underground last week will likely push the country closer toward a fully functional nuclear arsenal.
North Korea's UN mission circulated a report today from the country's news agency saying North Korean scientists and technicians "are in high spirit to detonate H-bombs ... capable of wiping out the whole territory of the US all at once as it persistently moves to stifle the DPRK."
The country's official name is the Democratic People's Republic of Korea.
More From This Section
The Security Council pledged immediately after the January 6 announcement of the nuclear test to swiftly pursue new sanctions.
It strongly condemned the test as a "clear violation" of previous UN sanctions resolutions.
The council last approved sanctions against North Korea after its third nuclear test on February 12, 2013.
That resolution was largely negotiated by the United States and China, North Korea's traditional ally.
The council diplomat said the United States, which is leading the current negotiations, is moving forward "in a careful, thorough and deliberate way," consulting closely with China but also with other council members, including Japan.
The diplomat said a new resolution isn't expected immediately, likely not in less than three weeks.