North Korea may be preparing another ballistic missile test after Japan detected suspicious radio signals coming from the secretive state.

Although such signals are not unusual and satellite images did not show fresh activity, Japan’s government has been put on alert.

“North Korea might launch a missile within the next few days,” a source told the Japan Times.

After firing rockets two to three times a month since April, North Korean tests paused after a 15 September launch of a ballistic missile over northern Japan.

But analysts suspect North Korea could resort to more military provocations after Donald Trump put Pyongyang back on a list of state sponsors of terrorism on 20 November.

However, a source told Reuters the signals were "not enough to determine” if a launch is likely soon.

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Japan's Kyodo news agency reported that the signals might be related to winter military training by the North Korean military instead.

Kim Jong-un, North Korea's leader, is pursuing nuclear weapons and missile programmes in defiance of UN Security Council sanctions and has made no secret of plans to develop a missile capable of hitting the US mainland. The country 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.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

"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 US alliance remains strong and capable of countering any North Korean provocations or attacks."

Two US 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 US government would not be surprised if such a test were to take place in the very near future.

Other US 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 US 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 faster than expected in developing its arsenal.