WASHINGTON: US presidential candidates and members of the US Congress demanded more sanctions on North Korea on Wednesday after its latest nuclear test, but major powers will likely be reluctant to take the tough steps necessary to force Pyongyang to abandon its weapons programme, former US officials and analysts said.

North Korea is already under a wide array of international sanctions, and diplomats said UN Security Council members were expected to discuss the possibility of adding to these in coming days. But steps taken so far stop short of the all-out economic offensive that forced Iran to the nuclear negotiating table.

Asia analysts said China would likely support more UN sanctions, even though it is North Korea's neighbor and main ally, but within limits, for fear of destabilising what has long been a physical buffer between it and US-backed South Korea.

Washington, too, has been cautious. While US sanctions have aggressively targeted Pyongyang's military and weapons programme, the United States has not imposed crippling economic sanctions, in part because these would hit Chinese firms and banks that do the vast bulk of business with North Korea, former US officials said.

"We are deeply interlinked and if you hold an economic gun to China's head, you are holding it to your own head," said Joseph DeThomas, a former US diplomat who worked on sanctions on North Korea and Iran, referring to the close economic relations between the world's two largest economies.

Republican front-runner Donald Trump urged China to rein in its ally or face trade repercussions, while his main Democratic rival, Hillary Clinton, said the United States should tighten sanctions on North Korea and called on Beijing to be more assertive in deterring Pyongyang's "irresponsible actions."

US State Department spokesman John Kirby said Washington would work with the international community on an "appropriate response" to Pyongyang. He said this would be "measured, deliberate, tough, clear and concise."

An adviser to Japanese Prime Minister Shinzo Abe said Tokyo had begun discussing a new UN sanctions resolution with Washington.

Katsuyuki Kawai told Reuters one option for Japan itself could be to reimpose bilateral sanctions it eased in 2014 in return for North Korea's reopening of a probe into the status of abducted Japanese citizens.

DeThomas, the former diplomat, said China could exert more pressure on North Korea by restricting energy supplies and investment in areas such as minerals and mining. It could also restrict informal border trade, or even take a different approach to North Korean refugees - allowing them in rather than shutting them out.

In 2013 China cut crude oil exports to North Korea as an apparent punishment for an earlier nuclear test.

But DeThomas said any discussions on sanctions at the United Nations would go nowhere close to the steps necessary to effect change in North Korea.

"From China's perspective, North Korean nuclear weapons are a bad thing, but the collapse of the North Korean regime would be a worse thing," DeThomas said.

Zhang Liangui, an expert on North Korea at the Central Party School in Beijing, which trains rising Chinese officials, said it was hard to see how effective any new sanctions would be on a country already all but isolated from the rest of the world.

"The North Korean nuclear issue has been going on for two decades now, and even with the repeated condemnation of the international community North Korea has continued to make progress on its nuclearisation," Zhang said.