After firing missiles at a pace of about two or three a month since April, North Korean missile launches paused in September, after Pyongyang fired a rocket that passed over Japan's northern Hokkaido island.
"This is not enough to determine (if a launch is likely soon)," the source told Reuters.
Japan's Kyodo news agency reported late on Monday that the Japanese government was on alert after catching such radio signals, suggesting a launch could come in a few days. The report also said the signals might be related to winter military training by the North Korean military.
South Korea's Yonhap news agency, citing a South Korean government source, also reported that intelligence officials of the United States, South Korea and Japan had recently detected signs of a possible missile launch and have been on higher alert.
Asked about the media reports, Pentagon spokesman Colonel Robert Manning told reporters the United States continued to watch North Korea very closely.
"This is a diplomatically led effort at this point, supported by military options," he said.
"The Republic of Korea and U.S. alliance remains strong and capable of countering any North Korean provocations or attacks."
Two U.S. government sources familiar with official assessments of North Korean capabilities and activities said that while they were not immediately familiar with recent intelligence suggesting that North Korea was preparing to launch a new missile test, the U.S. government would not be surprised if such a test were to take place in the very near future.
Other U.S. intelligence officials noted North Korea has previously sent deliberately misleading signs of preparations for missile and nuclear tests, in part to mask real preparations, and in part to test U.S. and allied intelligence on its activities.
(Reporting by Nobuhiro Kubo in TOKYO, Phil Stewart, Mark Hosenball and John Walcott in WASHINGTON, and Soyoung Kim in SEOUL; Writing by Linda Sieg; Editing by Lincoln Feast)
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