"We are in a very difficult and very tough phrase of negotiation," Hill told CNN ahead of his meeting with North Korean officials.
The latest dispute centers on measures to verify that North Korean has disabled its Yongbyon nuclear reactor and reprocessing facilities that produce plutonium for nuclear weapons.
The United States has insisted on verification before removing North Korea from the U.S. list of nations that sponsor terrorism.
"The North Koreans invited Chris Hill to come, so we hope that there is some effort to address the verification protocol, because that's what we need," said Secretary of State Condoleezza Rice.
The impasse comes amidst unconfirmed reports that North Korean leader Kim Jong Il may be seriously ill.
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