The top US nuclear negotiator said it is imperative to find a solution to the deadlock over North Korea's nuclear program ahead of fresh talks between the two sides Tuesday. "I am not running out of energy. But we really have to make some progress. We can't afford any further delays here," US Assistant Secretary of State Christopher Hill told reporters. But Hill and his North Korean counterpart, vice foreign minister Kim Kye Gwan, sounded notes of caution over how much progress can be made in their Singapore talks. Hill and Kim arrived separately on Monday for the daylong talks that will aim to break a deadlock over how the communist country will account for its nuclear program's hazy past. The United States says North Korea missed a Dec. 31 deadline to provide a full inventory of its nuclear activities and facilities, halting progress in the six-nation talks being hosted by China. North Korea says it provided a list in November, but Washington says it was incomplete.