TOKYO/WASHINGTON (Reuters) - Japan has detected radio signals suggesting North Korea may be preparing another ballistic missile launch, although such signals are not unusual and satellite images did not show fresh activity, a Japanese government source said on Tuesday.

A flag is pictured outside the Permanent Mission of North Korea in Geneva, Switzerland, November 17, 2017. REUTERS/Denis Balibouse

After firing missiles at a pace of about two or three a month since April, North Korean missile launches paused in September, after it 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.

North Korea is pursuing its nuclear weapons and missile programmes in defiance of U.N. Security Council sanctions and has made no secret of its plans to develop a missile capable of hitting the U.S. mainland. It has fired two missiles over Japan.

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.

South Korean Unification Minister Cho Myoung-gyon told reporters on Tuesday there have been “noteworthy” movements from the North since its last missile launch in mid-September, but there was no hard evidence of another nuclear or missile test.

“North Korea hasn’t been engaging in new nuclear or missile tests but recently we’ve seen them persistently testing engines and carrying out fuel tests,” said Cho at a media event in Seoul.

“But we need some more time to see whether these are directly related to missile and nuclear tests.”

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.

South Korea’s Cho said North Korea may announce the completion of its nuclear programme within a year, as it is moving more faster than expected in developing its arsenal.

North Korea defends its weapons programmes as a necessary defence against U.S. plans to invade. The United States, which stations 28,500 troops in South Korea, a legacy of the 1950-53 Korean war, denies any such intention.