North Korea failed to meet a Saturday deadline for shutting down its main nuclear reactor, with a key US official saying that the failure was draining momentum from a disarmament plan for the North. The United States and other governments involved in six-nation talks on North Korea's nuclear program called on Pyongyang to abide by the two-month-old agreement that set a timetable for its nuclear disarmament. Assistant Secretary of State Christopher Hill, the chief US negotiator, sounded a worried note that continued North Korean foot-dragging could imperil the disarmament plan. "We don't have a lot of momentum right now. That is for sure," Hill told reporters before conferring with his Chinese counterpart, Wu Dawei, after meetings earlier in the week with South Korean and Japanese negotiators.