North Korea will be the "most intractable" issue for the next US president, a former Swedish prime minister said, calling for greater efforts to enlist China's help in reining in the recalcitrant regime.

Carl Bildt, who served as Sweden's prime minister from 1991 to 1994 and as foreign minister from 2006 to 2014, made the remark in a recent article to the Project Syndicate, warning that Pyongyang is "slowly but surely progressing toward a deliverable nuclear weapon."

"While the crisis in Syria may be the most urgent conflict the next US president will have to address, developments in North Korea could well turn out to be the most intractable," he said, noting the North's two nuclear tests and a series of missile launches.

Bildt stressed that the threats posed by the North "cannot be mitigated without China."

"The US must reach out to China to explain the nature of the system and discuss measures that might address its fears," Bildt said.

"The ultimate goal of talks should be to secure China's cooperation in developing a comprehensive peace treaty, signed by all regional powers, that normalizes relations with a North Korea that agrees to give up its nuclear weapons and commit to political reform," he said.

The US should also engage China in talks about the aftermath of the North's potential collapse, he said.

"Discussions must also address what might happen if North Korea's regime suddenly collapses -- a possibility, though not imminent, that cannot be excluded. In such a scenario, a plan -- agreed in advance by the US, China, South Korea, and Japan -- that covers border control, refugees, port access, and military operations would need to be in place," he said.

"Without China's buy-in, efforts to curb North Korea's nuclear ambitions are likely to falter. In that case, the country's volatile leadership could eventually possess long-range nuclear capabilities -- an outcome that must be averted at all costs. The last thing a dangerous and uncertain world needs is more danger and uncertainty," he said. (Yonhap)