China's coal imports from North Korea have surged in recent months, government data showed Monday, raising questions about Beijing's commitment to international sanctions intended to curb Pyongyang's nuclear programme.

As the isolated country's sole ally and main provider of trade and aid, Beijing's participation in the UN-imposed restrictions is crucial for their success.

But in August China imported 2.465 million tonnes of North Korean coal worth $113 million, 60 percent more than in April when sanctions were imposed and 35 percent more than a year ago, according to data on the Customs website.

Imports of North Korean iron ore also rose, from 110,500 tons in April to 197,000 tons last month, data showed.

The UN Security Council in March agreed to impose sanctions on certain North Korean exports, including coal and iron, But it also said it would allow trade to continue for "livelihood" purposes -- if the proceeds did not go towards funding Pyongyang's nuclear or weapons programmes.

The UN did not set criteria for making that determination, leaving each country to make its own decision.

In April China announced it would place restrictions on imports of both items, among others.

Trade with the world's second largest economy is crucial for the isolated and impoverished North, which has suffered regular food shortages and an outright famine in the mid-1990s.

In 2015 China accounted for more than 90 percent of North Korea's $6.25 billion in total trade, according to figures from South Korea's state-run Korea Trade-Investment Promotion Agency.

China's total imports from North Korea in August rose 16.7 percent year-on-year to $291.3 million, Customs data showed.

The figures come just weeks after North Korea tested its fifth and most powerful nuclear device.

China has said it "firmly opposes" the test, but analysts believe it has resisted targeting the North's fragile economy for fear of provoking the regime's collapse.

It fears this could prompt a flood of cross-border refugees and ultimately the prospect of US troops stationed on its border in a reunified Korea.

The international community has engaged in a flurry of diplomacy to try to persuade China to use its leverage with Pyongyang.