Christopher Hill, who served as a chief U.S. negotiator in the six-party talks to rid North Korea of its nuclear weapons, also called on China's top leadership to wield more influence over the North to prevent it from conducting such a test.
Signs have emerged that North Korea may be readying for a nuclear test following its failed launch of a long-range rocket this month, which drew swift condemnation from the United Nations Security Council, of which China is a member.
Pyongyang's previous unsuccessful launches of long-range rockets in 2006 and 2009 were followed by nuclear tests of plutonium devices.
"I think it is correct to be concerned that the next nuclear test will be somehow different and bigger than previous nuclear tests," Hill told Yonhap News Agency in an interview in Seoul.
If North Korea conducts a third nuclear test, Hill said it would be "an effort to really be belligerent" and hostile to its neighbors.
Hill urged China, North Korea's sole major ally, to have a sense of urgency in dissuading the isolated state from doing so, but partly admitted that Beijing may have a limited influence over Pyongyang.
"I hope the Chinese to come to understand that they have been a big loser in this whole process," Hill said on the sidelines of a security forum in Seoul.
"I think a nuclear test really requires the Chinese response not just from its foreign ministry but from real power setters in China. They should tell North Korea not to do it."
Former U.S. secretary of state Christopher Hill (Yonhap file photo)
By launching the rocket, North Korea reneged on a Feb. 29 deal with the U.S. that had spawned some hope of diplomacy with the North under new leader Kim Jong-un, who inherited the impoverished state following the death of his father Kim Jong-il in December.
Under the "Leap Day" deal, North Korea pledged to suspend nuclear and missile tests and halt uranium enrichment under supervision of U.N. inspectors. In return, the U.S. promised 240,000 tons of food aid, but the April 13 launch scuppered the deal.
Analysts have puzzled over why North Korea recanted on the deal and the surrounding circumstances have raised questions about how much control the young Kim has over the military.
Hill, now dean of the Josef Korbel School of International Studies at the University of Denver, said he was "really not sure who is running that country."
"We know the tone of negotiations (between the U.S. and North Korea) was very positive. The Chinese had worked very hard for several months to put this together," Hill said.
"And yet a few weeks later, North Korea reneged on the deal" although nothing had gone bad, he said.
"North Korea has a bit of confusion at the top. People at the top didn't uphold the agreement. Whether the confusion is between civilian and military authorities, I can tell you it didn't make sense."
Hill brokered some important denuclearization deals with North Korea, including the Sept. 19, 2005, Joint Statement, through a series of bilateral and six-way negotiations that also involved South Korea, China, Russia and Japan.
His accomplishments, however, were overshadowed by the North's nuclear tests. The six-way talks have been on hold since 2009, when North Korea walked out of the process after a new round of U.N. sanctions.