"The nuclear issue is now at a watershed of whether it goes to a stable phase or undergoes a squeaking and bumpy" phase, he said.
As part of the disarmament deal agreed in February, North Korea committed to revealing everything about its pursuit of nuclear weapons, including all facilities, activities and material by the end of the year.
Song's comments came as the chief US nuclear negotiator to North Korea said the disabling of the country's main plutonium-producing reactor was going ahead smoothly, although he conceded there were "definitely some differences" over the declaration process.
Christopher Hill was speaking at the end of a rare visit to North Korea, during which he toured the country's Yongbyon nuclear facility.
"We're not looking for some sort of cliffhanger"
Christopher Hill, chief US envoy
"The disablement activities are going well and on schedule," Hill said on Wednesday before leaving Pyongyang for Beijing. "I'm satisfied with the results."
Hill's arrival in the Chinese capital had originally been expected to herald the start of a fresh round of six-nation talks, during which envoys would review North Korea's progress in declaring its nuclear programmes.
The talks bring together negotiators from the US, China, Russia, Japan and the two Koreas.
However on Tuesday a spokesman for the US state department said the talks had been postponed due to "scheduling and logistics" issues.
It is not clear when next round of talks, usually held in Beijing, will take place.
Hill spent three days in North Korea, where he met his counterpart, Kim Kye Gwan, the vice foreign minister, and visited the Yongbyon nuclear complex including the reactor, reprocessing plant and fuel fabrication plant.
He said officials at Yongbyon were "very cooperative" with US experts supervising the disablement procedures, and he believed that the disablement would be finished on time.
"We're not looking for some sort of cliffhanger," he said.
"What we want to see is that this is going on as quickly as possible and as safely as possible, and we are very much convinced that that is the case."
Speaking in Beijing later, Hill said differences over North Korea's accounting of all its nuclear programmes must be resolved in order to meet the goal of total disablement by the end of December.
He said the declaration should include "all the facilities, materials and programmes" in North Korea's possession.
"It is important that the declaration, even as a first draft, should be complete and correct."
There has been little official indication as to what was causing the delay in the North's declaration, although speculation has been focusing on disagreements over North Korea's alleged uranium enrichment program.
In 2002 the US accused North Korea of seeking to secretly enrich uranium in violation of an earlier disarmament deal, sparking the latest nuclear standoff.
Another reason for the delay could be disagreements over the amount of plutonium the North admits it has produced.
Last month North Korea began disabling the plutonium-producing Yongbyon reactor, which was shut down in July, and two other facilities under US supervision.
Pyongyang has been promised the equivalent of one million tonnes of fuel oil, economic aid and political concessions for disarming under the February nuclear pact.