North Korea drew criticism from major ally China on Friday after the failed launch of what analysts say was an intermediate-range ballistic missile in an embarrassing setback for leader Kim Jong-un.

The test-launch was attempted as the reclusive country celebrated the birthday of Kim's grandfather. It followed the North's fourth nuclear test in January, and a long-range rocket launch in February that led to new UN sanctions.

China - North Korea's most important economic and diplomatic backer - has been angered by Pyongyang's nuclear tests and rocket launches in the face of United Nations sanctions that Beijing has also backed.

Chinese foreign ministry spokesman Lu Kang said the UN Security Council was clear on North Korean rocket launches.

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"At present, the situation on the peninsula is complex and sensitive," Lu told reporters.

"We hope all parties can strictly respect the decisions of the Security Council and avoid taking any steps that could further worsen tensions."

Chinese state media was more direct.

"The firing of a mid-range ballistic missile on Friday by the Democratic People's Republic of Korea [DPRK], though failed, marks the latest in a string of sabre-rattling that, if unchecked, will lead the country to nowhere," China's official Xinhua news agency said in an English-language commentary.

"Nuclear weapons will not make Pyongyang safer. On the contrary, its costly military endeavours will keep on suffocating its economy."

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Friday was the anniversary of North Korean founding president Kim Il-sung's birthday, which is widely celebrated. In 2012, it was marked by a long-range rocket launch attempt that also failed.

The US Pentagon described Friday's missile launch as a "catastrophic" failure after the missile reportedly exploded in the air just a few seconds after liftoff.

Defense Secretary Ashton Carter said the launch was "another provocation by North Korea in a region that doesn't need that kind of behaviour".

As North Korea intensifies testing of its ballistic missile technology, a US website said Friday it also sees further signs from satellite imagery that Pyongyang is looking to produce more plutonium for nuclear weapons.

The website 38 North, which monitors sites in North Korea associated with its weapons programmes, said an image taken Monday at the Nyongbyon nuclear complex shows a rail flatcar at radiochemical laboratory complex where the North separates weapons-grade plutonium from waste from a nuclear reactor.

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It said the tanks or casks seen on the flatcar could be used to supply chemicals or haul out waste products. In recent weeks, exhaust plumes have been seen at the laboratory, also suggesting nuclear reprocessing activity could be in the works.

"The presence of a loaded flatcar, together with the presence of exhaust plumes, suggest that North Korea is preparing or conducting a reprocessing campaign to separate more plutonium for weapons," said the analysis by Joseph Bermudez, a specialist in satellite imagery and North Korea's military.

Last week, the Institute for Science and International Security, a Washington-based think-tank, said there is growing indications that North Korea is separating plutonium, but said it is hard to say with certainty. It estimated the reactor could have produced about 5kg to 7kg of weapon-grade plutonium since its 2013 restart - enough for one to three nuclear weapons.

Pyongyang is already believed to have a handful of crude nuclear bombs and is making progress towards having a nuclear-tipped missile that could reach the US mainland.