"First of all, we don't comment on intelligence matters," said State Department spokesman Robert Wood.
"But North Korea's missile activities and, you know, missile programs are a concern to the region. There's no secret there. And a ballistic missile launch by North Korea would be unhelpful and, frankly, provocative."
Earlier a US counter-proliferation official said there were signs North Korea was preparing to test fire its longest-range missile, the Taepodong-2.
"But whether it will carry out the launch or not is entirely unclear, as is the timing for a possible launch," said the official, who spoke to AFP on condition of anonymity.