"We cannot predict if or when the multilateral talks will take place because North Korea has not made any response," Yoon Young-kwan told reporters.
China, North Korea's closest ally, has been acting as an intermediary to arrange another round of talks between Washington and Pyongyang similar to those Beijing hosted in April.
Yoon said discussions between Beijing and Pyongyang "were slowing down a little."
"The ball is in North Korea's court," said Yoon, adding that the main issue is whether North Korea agrees to broaden the talks after holding a new round of three-way talks.
In the past few days, South Korean officials have expressed optimism that the United States, North Korea and China will meet in Beijing soon, possibly next month, to discuss how to end the nine-month-old crisis over North Korea's suspected development of nuclear weapons.
U.S. and North Korean officials have met only once since the crisis began last October.
The United States considers the North's nuclear ambitions to be a regional threat and has insisted on multilateral talks that would include South Korea and Japan, two key U.S. allies in the region, and possibly Russia.
North Korea wants a one-on-one meeting with Washington, calling it a bilateral matter, and says it will not give up its nuclear ambitions unless the United States signs a nonaggression treaty and promises economic aid.
A U.S. envoy said Monday that China has done all it can diplomatically to get North Korea to resume nuclear talks, but continues to have leverage as the impoverished North's major source of food and fuel aid.
"We've considered those efforts very important," Bolton, the State Department's top arms-control official, told reporters. "I'm not sure that there's anything else specifically that we could think of that the government here could do that they haven't already tried."
However, he noted that China also has leverage over the North's Stalinist dictatorship as the major supplier of food and fuel aid to its decrepit economy.
"That's a point we've made in our discussions with China many times," he said.
China has given no public indication that it would be willing to use aid to pressure North Korea. Analysts suggest that Beijing might be afraid of the consequences of aid cutbacks, including a possible influx of refugees if the North's already weak economy falters further.
Bolton reaffirmed U.S. insistence on including Japan, South Korea and Russia in a new
round of talks.
"I don't see that we're making progress by denying entry into the process of countries that have a legitimate and substantial stake in the outcome," he said.
And he suggested that the U.N. Security Council might have a role to play in the dispute – a route that China has opposed.
"Not being able to address the North Korean issue would be a grave impairment of the Security Council," he said.
The American envoy, who was in Beijing as part of a regular series of U.S.-Chinese meetings on security, was due to fly to South Korea on Tuesday and then visit Japan.
The nuclear crisis erupted in October when an American envoy claimed North Korean officials told him they had a secret uranium-processing program. The U.S. then suspended fuel shipments that had been made under a 1994 deal in which North Korea gave up seeking nuclear weapons in return for aid.
Retaliating for the fuel cutoff, North Korea expelled foreign inspectors and vowed to restart its Yongbyon plant for reprocessing spent nuclear fuel into plutonium for weapons.
North Korea, which is believed to have one or two nuclear devices, recently said it had completed reprocessing that fuel. U.S. intelligence could not confirm that.