According to the official, a test launch could happen without North Korea issuing a standard notice to commercial aviation and maritime shipping warning them to stay away from the missile's path.
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"We hope they issue a notification, but at this point we don't expect it. We are working on the assumption they won't," he said.
He said the launch could be "imminent," but also cautioned that the United States "simply doesn't know."
An Obama administration official told Reuters "our working assumption is that there are two missiles that they may be prepared to launch". That was in line with South Korean media reports.
South Korea said on Wednesday it has asked China, North Korea's only major ally, to rein in the hermit state and has raised its surveillance after the North moved at least one long-range missile in readiness for a possible launch.
Admiral Samuel Locklear, the commander of US forces in the Pacific region, said the US military believed North Korea had moved an unspecified number of Musudan missiles to its east coast.
The Combined Forces Command in Seoul raised its "Watchcon 3" status, a normal defence condition, by one level in order to step up monitoring and increase the number of intelligence staff, a senior military official told the Yonhap news agency in the South on Wednesday.
"There are clear signs that the North could simultaneously fire off Musudan, Scud and Nodong missiles," Yonhap quoted an unidentified official as saying.
Meanwhile, some Chinese travel agencies cancelled tours to North Korea due to safety concerns. However, other Chinese operators say they still plan to take tourists in to the country.
North Korea has warned foreigners in South Korea to evacuate to safe places due to the threat of war and it has advised foreign diplomats in Pyongyang that it can't guarantee their safety past Wednesday.
AP contributed to this report
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