The United States has responded to North Korea's third test of an intercontinental ballistic missile, its most powerful yet, with a call for all nations to cut off diplomatic and trade ties with Pyongyang.

But events in North Korea are moving faster than the time it would take for sanctions to take effect, calling into question the viability of this strategy.

US Ambassador to the United Nations Nikki Haley indicated at an emergency session of the United Nations Security Council yesterday that President Donald Trump had also asked Chinese President Xi Jinping to cut off oil to North Korea - a move that would severely hurt the North Korean economy and its people.

"We need China to do more," she said. "Through sanctions we have cut off 90 per cent of North Korean trade and 30 per cent of its oil. But the crude oil remains. The major supplier of that oil is China."

However, as Dr Denny Roy, senior fellow at the East-West Centre in Hawaii, said: "The amount of time remaining for even tough sanctions to have a chance to take effect is very limited."

"Even with China doing more than ever before, and even with Washington implementing secondary financial sanctions, my guess is Pyongyang is not giving consideration to capitulating but, rather, is recommitted to reaching the goal as fast as possible. I doubt that even a total cut-off of oil supplies from China would change the outcome," he told The Straits Times in an e-mail.

Similarly, Dr Carlyle Thayer, emeritus professor at the University of New South Wales at the Australian Defence Force Academy, said: "We must avoid unrealistic expectations that an immediate clamp of sanctions will lead (North Korean leader) Kim Jong Un to back down."

"Trump declared that the era of 'strategic patience' was over, but the context has changed. It is now time to give UN sanctions a chance and adopt strategic patience, and not respond to every North Korean provocation with a call for further immediate sanctions," he told The Straits Times in an e-mail.

"China and, to a lesser extent, Russia are the key," he said. China is North Korea's most important trading partner and Pyongyang is thought to rely on Beijing for much of its oil supplies. Russia, which shares a short land border with North Korea, also has growing trade and labour ties with it. "If they move beyond UN sanctions, (North Korea) would be hurt badly over time," Dr Thayer added.

A possible path ahead for the US is to interdict North Korean shipping, or prevail on its allies to do the same, analysts say. Thus far, Washington has stopped short of such measures. In September, the UN Security Council passed a resolution requesting member nations to inspect ships going in and out of North Korean ports, but it stopped short of an American proposal to authorise the use of force to do so.

However, a State Department official told ABC News the US was working with partner countries at the UN and in one-on-one conversations to increase maritime interdiction.

The Treasury Department could also target banks that do business, even indirectly, with North Korean entities. Mr Joshua Stanton, a Washington-based attorney and expert on sanctions on North Korea, told The Straits Times: "The Treasury and Justice departments should intensify their investigations... to identify and, where necessary, impose heavy penalties on banks that fail to do due diligence, and keep Kim Jong Un's money frozen in place. We haven't seen it done to North Korea yet."

Experts are also cautious over diplomatic initiatives to ease the tensions and avoid escalation.

Analysts point out that Pyongyang will never give up its nuclear weapons in the face of what it sees as an existential threat from the US. But the US insists only on denuclearisation.

In the face of this stalemate, pushing North Korea into dire economic straits with sanctions may even provoke Pyongyang, some analysts have warned.

Reflecting the divide in Washington, Mr Anthony Ruggiero, a senior fellow at the Foundation for the Defence of Democracies, wrote yesterday in an article for Fox News: "Critics will inevitably complain that economic sanctions the US has slapped on North Korea since President Trump took office are failing to stop the North from making progress on its missile and nuclear weapons."

But "such criticism is premature", he wrote. "There was no way that 10 months of getting tough on North Korea by the Trump administration could compensate for more than 10 years of paralysis and concessions by previous American presidents. Even the toughest sanctions need time to work."

"The Trump administration has not gone far enough and continues to pull its punches against Chinese banks that are the key to North Korea's continued sanctions evasion," he wrote.

"Perhaps President Trump fears Chinese retaliation if he takes a stronger stand."