Speaking Friday at a news conference in Tokyo -- the day after it was reported satellite photos taken in late August detected steam rising from a building near a reactor at the Yongbyon, North Korea, nuclear complex -- Davies said the matter was being watched carefully.
The complex was shut down in 2007.
"If it turns out these reports are true, that North Korea has started the 5 megawatt plutonium reactor, this would be a very serious matter, we think a misstep on the part of North Korea, because of course, it would violate a series of U.N. Security Council resolutions."
A move to reopen the facility was in violation of international law and North Korea's own commitments and promises, he said.
Davies, the U.S. State Department's special representative for North Korean policy, visited Beijing and Seoul before arriving in Tokyo.
After meeting with South Korean officials in Seoul he said Washington would not agree to the resumption of negotiations "until we see a much greater degree of willingness" on the part of North Korea to end its nuclear program, Yonhap News reported.