China says it will reduce aid to North Korea if Pyongyang goes ahead with a planned nuclear test.

Chinese state-run media issued the warning after North Korea threatened to carry out a third nuclear test and more rocket launches in response to tightened United Nations sanctions.

"If North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance to North Korea," the Global Times, which is close to the ruling Communist Party, said in an editorial.

"China hopes for a stable peninsula, but it's not the end of the world if there's trouble there," it added.

"We should have a pragmatic attitude to deal with the problems and pursue the optimal ratio between our investment of resources and strategic gains."

China is North Korea's only major ally and its main energy supplier and trading partner.

On Friday, North Korea changed the focus of its anger over the new sanctions from the United States to South Korea.

It vowed to use "physical counter-measures" if South Korea participates in them.

When Pyongyang announced on Thursday it would carry out a third nuclear test, without specifying when, Beijing called for restraint and dialogue.

Foreign ministry spokesman Hong Lei said China hoped all parties would "stay calm, be discreet in words and deeds and look at the long-term interest and push for the resumption of the six-party talks".

The talks are chaired by China and also involve the two Koreas, the United States, Japan and Russia.

Their aim has been to convince Pyongyang to abandon its nuclear program in return for aid and security guarantees, but the process has been moribund since the North abandoned the forum in 2009.

AFP/ABC