Geneva: North Korea’s latest test of an atomic weapon leaves the United States with an uncomfortable choice: stick with a policy of incremental sanctions that has clearly failed to stop the country’s nuclear advances, or pick among alternatives that range from the highly risky to the repugnant.

A hard embargo, in which Washington and its allies block all shipping into and out of North Korea and seek to paralyse its finances, risks confrontations that allies in Asia fear could quickly escalate into war. But restarting talks on the North’s terms would reward the defiance of its young leader, Kim Jong Un, with no guarantee that he will dismantle the nuclear programme irrevocably.

For more than seven years, President Barack Obama has sought to find a middle ground, adopting a policy of gradually escalating sanctions that the White House once called “strategic patience.”

But the test on Friday — the North’s fifth and most powerful blast yet, perhaps with nearly twice the strength of its last one — eliminates any doubt that that approach has failed and that the North has mastered the basics of detonating a nuclear weapon.

Despite sanctions and technological backwardness, North Korea appears to have enjoyed a burst of progress in its missile programme over the past decade, with experts warning that it is speeding toward a day when it will be able to threaten the US West Coast and perhaps the entire country.

“This is not a cry for negotiations,” said Victor Cha, who served in the administration of President George W. Bush and now is a North Korea expert at the Centre for Strategic and International Studies in Washington. “This is very clearly a serious effort at amassing real nuclear capabilities that they can use to deter the US and others.”

Cha said the usual response from Washington, Seoul and Tokyo — for another round of sanctions — was not likely to be any more successful at changing the North’s behaviour than previous rounds. That means Obama’s successor will confront a nuclear and missile programme far more advanced than the one that Obama began grappling with in 2009.

Obama has refused to negotiate with the North unless it agrees first that the ultimate objective of any talks would be a Korean Peninsula without nuclear arms. But Kim has demonstrated, at least for now, that time is on his side. And as he gets closer to an ability to threaten the United States with a nuclear attack, and stakes the credibility of his government on it, it may be even more difficult to persuade him to give up the programme.

In a statement on Friday, Obama condemned the North’s test and said it “follows an unprecedented campaign of ballistic missile launches, which North Korea claims are intended to serve as delivery vehicles intended to target the United States and our allies”.

“To be clear, the United States does not, and never will, accept North Korea as a nuclear state,” he said.

Many experts who have dealt with North Korea say the United States may have no choice but to do so.

“It’s too late on the nuclear weapons programme — that is not going to be reversed,” William Perry, the defence secretary under president Bill Clinton during the 1994 nuclear crisis with North Korea, said in August at a presentation in Kent, Connecticut.

The only choice now, he argued, is to focus on limiting the missile programme.

Yet the latest effort to do that, an agreement between the United States and South Korea to deploy an advanced missile defence system in the South, has inflamed China, which argues the system is also aimed at its weapons. While US officials deny that, the issue has divided Washington and Beijing so sharply that it will be even more difficult now for them to come up with a joint strategy for dealing with the North.

The breach between China and the United States was evident during Obama’s meeting with President Xi Jinping last weekend.

“I indicated to him that if the THAAD bothered him, particularly since it has no purpose other than defensive and does not change the strategic balance between the United States and China, that they need to work with us more effectively to change Pyongyang’s behavior,” Obama said, referring to the Terminal High Altitude Area Defence system, as the advanced missile defense project is known.

But Obama noted that sanctions had failed at having much effect. That is largely because the Chinese have left open large loopholes that have kept the North Korean economy alive and, by some measures, enjoying more trade than anytime in years.

In a recent paper, two researchers concluded that sanctions so far “have had the net effect of actually improving” North Korea’s procurement capabilities for its weapons programme. To evade sanctions, the North’s state-run trading companies opened offices in China, hired more capable Chinese middlemen and paid higher fees to employ more sophisticated brokers, according to Jim Walsh and John Park, scholars at MIT and Harvard respectively.

The sanctions, Cha noted, “are supposed to inflict enough pain so the regime comes back to the negotiation table, and that’s clearly not working; or it’s supposed to collapse the regime until it starves, and that’s not working either.”

“Unless China is willing to cut off everything, which they don’t appear willing to do, the sanctions may be politically the right thing to do and a requisite response, but they are not the answer to the problem,” he said.

That means the choices facing Obama’s successor will be stark. One option is to choke off all trade, in part by telling banks that conduct transactions with North Korea that they will be shut out of dealing in dollars around the world — an effective tactic against Iran before last year’s nuclear deal. But that would enrage the Chinese, and probably cut into cooperation on other issues.

At the same time, an attempt to intercept all shipping could quickly escalate into a full-blown conflict, something neither Obama nor the South Koreans and Japanese have been willing to risk.

On the other hand, reopening negotiations, which Donald Trump has indicated he is willing to consider, could mean paying North Korea again to freeze nuclear activities that the Bush administration and the Clinton administration had already rewarded them for stopping years ago.