"We have not achieved our goal," Christopher Hill told reporters.
"We had some real difficulty in consensus on moving forwards... in terms of coming up with the verification agreement, we don't seem to be narrowing differences," he said.
The six countries - the two Koreas, the U.S., China, Russia and Japan - have been discussing a Chinese-drafted verification protocol on means of probing North Korea's past nuclear activities.
Hill said it remained unclear whether the negotiations would continue tomorrow.
Currently the main stumbling block in the negotiations, which have been ongoing since 2003, is the U.S. demand for nuclear inspectors to be able to take samples from North Korean facilities out of the country for analysis.
The negotiations have been complicated by North Korea's refusal to acknowledge Japan as a participant over Tokyo's failure to meet its obligations under a 2007 six-party agreement to provide fuel aid to Pyongyang in exchange for the dismantling of North Korean nuclear facilities and disclosure of all information on past nuclear activities.
Earlier this week, the U.S. appeared to backtrack on a report classifying North Korea as a nuclear power.
"As a matter of policy, we do not recognize North Korea as a nuclear state," Department of Defense spokesman Stewart Upton said in a statement on Tuesday.
"What was contained in a recent Joint Forces Command report does not reflect official U.S. government policy regarding the status of North Korea."
The military report described North Korea as one of five Asian nuclear powers, along with China, India, Pakistan and Russia, saying: "North Korea has tested a nuclear weapon and has produced sufficient fissile material to create more such weapons."
