Assistant Secretary of State Christopher Hill made the comment Friday, in the South Korean capital of Seoul, where he was meeting with South Korea's Unification Minister Lee Jae-joung.
North Korea has agreed to disable its Yongbyon nuclear reactor and disclose its nuclear activities by the end of this year, in exchange for energy assistance and political concessions.
Hill says any aid South Korea has promised to Pyongyang must go hand in hand with denuclearization.
North Korea's official news agency says the country is fulfilling its commitment to shut down its nuclear program, and will "follow the moves of the U.S. and all other parties."
Hill is scheduled to travel to North Korea early next week to meet with senior officials and to observe the dismantling of the Yongbyon complex. American experts have been supervising the process since early this month.
He and envoys from the four other nations involved in the negotiations - Russia, China, South Korea and Japan - are expected to resume nuclear talks in Beijing by the end of next week. - VOA News