China is trying to punish its ally North Korea for its nuclear and missile tests, stepping up inspections of North Korean-bound cargo in an effort to send a message that it is piqued, without provoking the Pyongyang government.

Freight handlers and trading companies at ports and cities close to the North Korean border complain of more rigorous inspections and surprise checks that are raising the costs of doing business with an often unpredictable North Korea. Machinery, luxury goods and daily necessities such as rice and cooking oil are among the targeted products.

China is getting tough with an impoverished neighbour it has long supported with trade, aid and diplomatic protection for fear of setting off a collapse. The moves to restrict trade with North Korea come as Beijing falls under pressure to enforce UN sanctions passed after last month's nuclear test, Pyongyang's third.

Download the new Independent Premium app Sharing the full story, not just the headlines

Targeted in the sanctions are the bank financing and bulk smuggling of cash that could assist North Korea's nuclear and missile programmes as well as trade in the luxury goods that sustain the elite around the nation's leader, Kim Jong-un. Pyongyang has reacted with fury against South Korea and the US.

American officials in Beijing for two days of talks to lobby China on enforcement said on Friday they were heartened by Chinese expressions of resolve. Spurring Beijing to co-operate, the US officials said, is a concern that North Korean behaviour had begun threatening China's interests in a region vital to its economy and security. "There's reason to believe the Chinese are looking at the threat in a real way," David Cohen, an under secretary to the Treasury, told reporters.

China's change of tack with North Korea is not likely to foreshadow a total end to Beijing's support. North Korea remains a strategic buffer between China and a US-allied South Korea, and Chinese leaders worry that too much pressure could upend an already fragile North Korean economy and cause the Kim government to collapse, leaving Beijing with a security headache and possible refugee crisis.

But North Korea watchers said between blind support and abandonment there is much Beijing can do to try to rein in Pyongyang.

"We have to get away from the binary thinking that either they support North Korea or they pull the plug. That's not the way the world works," said Jonathan Pollack of the Brookings Institution, a Washington think-tank. "The interesting thing is not what happens at the UN but what the Chinese provide in economic aid and energy assistance."

Over the past decade, as sanctions have reduced trade and assistance to North Korea, China has stepped into the breach. By 2011, China provided nearly all of North Korea's fuel and more than 83 per cent of its imports, from heavy machinery to grain and electronics and other consumer goods. Though Pyongyang could look to other trading partners such as Russia, Iran or Kuwait for fuel and some goods, China's proximity makes it indispensable.

Chinese companies, often backed by the government, are enlarging North Korean ports and building roads, helping to underpin growth after more than a decade of famine and economic decay. But China has become disillusioned with Kim Jong-un. Since coming to power, Mr Kim has refused to heed Beijing's prodding to engage in economic reform and return to negotiations over the country's nuclear programme.

AP