North Korea has confirmed the restarting of a nuclear reactor seen as its main source of weapons-grade plutonium, raising the second red flag in 24 hours over its nuclear and ballistic missile programs.

The confirmation, by the head of the North's Atomic Energy Institute (AEI), came hard on the heels of suggestions by the chief of the national space agency of a possible satellite rocket launch next month.

Analysts said the two statements were clearly coordinated and partially aimed at raising tensions to place North Korea firmly on the agenda of an expected US-China summit later this month.

In an interview with the North's official KCNA news agency, the director of the AEI said all facilities at the Yongbyon nuclear complex — including a five-megawatt reactor — had "started normal operations".

North Korea mothballed the Yongbyon reactor in 2007 under a six-nation aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.

When fully operational, the reactor is capable of producing around six kilos of plutonium a year — enough for one nuclear bomb, experts say.

The AEI director said scientists had been "steadily improving" both the quality and quantity of the North's nuclear deterrent and he issued a by-now standard warning to the United States.

"If the US and other hostile forces persistently seek their reckless hostile policy ... [North Korea] is fully ready to cope with them with nuclear weapons any time," he said.

The warning followed strong hints from Pyongyang that it is considering a satellite rocket launch to mark the 70th anniversary of the founding of the ruling Workers' Party of Korea (WPK) on October 10.

Sorry, this video has expired The 'perpetual story' of Korean hostility.

"The world will clearly see a series of satellites... soaring into the sky at the times and locations determined by the WPK central committee," the director of the North's National Aerospace Development Administration said late on Monday.

The North insists its rocket launches are intended to put peaceful satellites into orbit. The United States and its allies see them as disguised ballistic missile tests.

North Korea's main diplomatic ally China has protected it from tougher sanctions over its nuclear and missile tests, but is seen as increasingly impatient with its provocative behaviour.

If the North does go ahead with a rocket launch, it would jeopardise an inter-Korean family reunion planned for October 20-26.

North wants attention from global community, experts say

Some analysts pointed to Chinese president Xi Jinping's scheduled state visit to the United States later this month as a reason for the North's recent statements.

"Pyongyang wants the North Korea issue high on the agenda of Xi's talks with president Obama," Koh Yu-Hwan, a North Korea expert at Dongguk University in Seoul, said.

"The North always wants the attention of the global community and knows that flagging its weapons programmes is one way to get it."

The North's only successful satellite launch was one using the Unha-3 rocket in December 2012 — a move that resulted in fresh sanctions and a surge in military tensions that culminated two months later in the North's third nuclear test.

South Korea, the United States and Japan all warned Tuesday that any fresh launch would violate UN resolutions banning the North from using ballistic missile technology.

Urging Pyongyang to refrain from what would be a "serious act of provocation," Seoul said it would not hesitate to press the UN Security Council for "a quick and effective response".

At the same time the South's defence ministry stressed that no activity had been observed to suggest a rocket launch was imminent.

Recent satellite images show the North has carried out significant upgrades of facilities at its main Sohae satellite launch station.

Analysts at the US-Korea Institute at Johns Hopkins University believe the Sohae launchpad can now handle rockets up to 50 metres (165 feet) in length — almost 70 per cent longer than the Unha-3 rocket.

The Unha-3 is seen as a prototype intercontinental ballistic missile (ICBM), although the North has yet to conduct a test showing it has mastered the re-entry technology required for an effective ICBM capability.

Developing a working ICBM would be seen as a game-changer, bringing the mainland United States within range of a possible nuclear strike.

Meanwhile, Japanese chief cabinet secretary Yoshihide urged North Korea to refrain from taking provocative action and said Japan would continue to cooperate with the United States and South Korea.

In 2012, North Korea launched what is generally considered a long-range rocket, putting what it said was a satellite into orbit.

Pyongyang called it a space launch vehicle, but the international community said it was a missile that violated UN Security Council resolutions.

AFP/Reuters