Biegun, who arrived in Seoul for a three-day visit on Sunday amid heightened tensions, also stressed that Washington will not give up, as Pyongyang has been threatening to take a "new way" if the US does not make concessions before the country's self-imposed year-end deadline, reports Yonhap News Agency.
"It's been a long year and we have not made nearly as much progress as we would have hoped. But we will not give up," Biegun told the medua after meeting with his South Korean counterpart Lee Do-hoon.
"Let me be absolutely clear. The US does not have a deadline. We have a goal to fulfil the commitments the two leaders made during their historic summit meeting in Singapore," he added.
He was referring to the first-ever summit between US President Donald Trump and North Korean leader Kim Jong-un in Singapore in June 2018.
Biegun also made an emphatic call for Pyongyang to return to the dialogue table, said Yonhap News Agency.
"It is time for us to do our jobs. Let's get this done. We are here and you know how to reach us," he said.
The US official also warned against any provocations by the North.
"We are fully aware of the strong potential for North Korea to conduct major provocations in the days ahead.
"To say the least, such an action will be most unhelpful in achieving the lasting peace on the Korean Peninsula. But it does not have to be this way," he added.
On December 7 and December 13, the North conducted what appeared to be rocket engine tests at its Dongchang-ri west coast satellite launch site, raising concerns that it could fire off a long-range rocket.
The North has been ramping up pressure on the U. to come up with an acceptable proposal to advance their stalled negotiations.
The US and the North last held working-level nuclear talks in Sweden in October. But the talks yielded little progress, with the North accusing the US of having come to the negotiating table "empty-handed".
( With inputs from IANS )