North Korea's main ally China says it "firmly opposes" Pyongyang's purported hydrogen bomb test and is monitoring the environment on its border with the North near the test site.

Foreign Ministry spokeswoman Hua Chunying told reporters that Wednesday's reported test was performed in defiance of the international community and urged North Korea to refrain from acts that might worsen tensions on the Korean Peninsula.

Hua said environment bureau officials were monitoring conditions near the border but that air quality near the bomb site was within the normal range.

While she made no mention of measures to respond to a test, analysts say Pyongyang's proceeding against Beijing's objections would seriously harm a relationship already under considerable strain. Despite that, China remains the North's most important important ally and trade partner.