South Korea and the United States are working together to determine the extent to which North Korea uses its workers abroad to raise money for its weapons of mass destruction programs, the U.S. special envoy for North Korean human rights issues said Tuesday.

North Korea's exports of workers has emerged as a new area the international community needs to address to tackle both the North's human rights abuses and its defiant pursuit of missile and nuclear weapons capabilities.

Speaking in an interview with Yonhap News Agency, Amb. Robert King acknowledged the difficulties of tracking the flow of money from North Korean workers to the regime in Pyongyang.

"It's very clear that North Korea uses a great deal of its resources for nuclear weapons, for missiles, for military equipment," he said at the U.S. Embassy in Seoul. "And to say that this dollar from this worker is going to a bomb, you don't have that kind of ability to account. It's a process that's happening and yes, we need to see what we can do to prevent it from happening."

Unofficial estimates put the number of North Korean workers abroad at 50,000, with most of them working in labor-intensive sectors in China and Russia, but also in Mongolia and the Middle East. The North Korean regime reportedly extorts between US$200 million and $300 million from these laborers annually.

As part of wide-ranging sanctions that punish Pyongyang for its recent nuclear and missile tests, U.S. President Barack Obama issued an executive order last month that among other things calls for sanctions against people who have engaged in the exportation of North Korean workers.

The measure has drawn attention as it could mean that Chinese and Russian firms hiring North Korean workers could be subject to the sanctions.

"At this point, we're beginning a process, and one of the things we're doing is looking for additional information, trying to make sure we know what's happening and where the workers are, what companies they are working for," King said. "We don't have a lot of information at this point. We're talking with the South Korean government and sharing information with them. We'll continue to consult with them."

Some of the information they need is which companies are hiring the workers, what goods they're producing and whether these products are being sold in the U.S.

King highlighted the increased international pressure on North Korea to improve its human rights record, especially following the 2014 U.N. Commission of Inquiry report that revealed atrocities amounting to "crimes against humanity." North Korea has been accused of holding hundreds of thousands of political prisoners in concentration camps, torture, rape and public executions -- all of which Pyongyang rejects as a U.S.-led campaign to topple its regime.

A key task that remains is pursuing accountability for those responsible for the crimes.

"I think that it's very clear that's where the policies of North Korea begin," King said, referring to North Korean leader Kim Jong-un. "So I think it's very clear that he's very responsible for many of the actions that go on. That doesn't mean he's solely responsible. He certainly sets a tone, but there are a lot of North Korean officials of one kind or another who are responsible."

The envoy listed heads of ministries, directors of departments and soldiers guarding the borders who are often reported to fire at North Koreans trying to flee the regime.

"All of them need to understand that these are violations of fundamental human rights, and we need to make an effort to hold them accountable," King said. "It's not enough to hold just the person at the top accountable. We need to hold people accountable from top to bottom."

Last month, the U.N. Human Rights Council adopted a North Korea human rights resolution that centers on appointing up to two independent experts to pursue accountability for the North's human rights abuses.

King arrived in Seoul Saturday and is scheduled to leave Wednesday after meetings with various senior officials at the Foreign and Unification Ministries and leaders of NGOs. (Yonhap)