In December, North Korea warned that a naval blockade would be considered an “act of war.” This week, foreign ministers from the US and about 20 other countries will discuss the possibility of intercepting vessels headed to North Korea, as part of expanding efforts to pressure Pyongyang into abandoning its nuclear weapons and ballistic missile programs.

The meeting will take place in Vancouver on Jan. 16, with Canada and the US serving as co-hosts. Attendees will primarily be the nations that sent troops to the Korean war of 1950-53, among them South Korea, Japan, Australia, India, Britain, and France. Neither China nor Russia will attend, though they will be given a readout of the discussion.

Beijing has criticized the meeting. The Trump administration said repeatedly last year that China, North Korea’s largest trade partner by far, has the leverage to apply meaningful pressure on Kim Jong-un’s regime if it wants. China has agreed to tighten UN sanctions against its neighbor. It recently reported a sharp drop in trade with the country. But China fears economic restrictions could get so tight that chaos, and a massive refugee crisis, breaks out on its doorstep.

Meanwhile, many observers doubt whether the trade restrictions enacted so far against North Korea, draconian as they are, actually create sufficient pressure to push the rogue regime on a different path. One obvious way to intensify pressure would be to intercept ships, which for the most part have continued to move freely in and out of the secretive country. Considering that North Korea has two long coasts, it would likely require many nations working together to intercept ships effectively.

“Maritime interdiction helps us to disrupt resources,” Brian Hook, director of policy planning for the US state department, told the AFP. “We will be discussing with our partners and allies the kind of steps that we can take on maritime interdiction and also to be cutting, disrupting funding and disrupting resources.”

There have been hints of the hidden maritime commerce that takes place between North Korea and other countries. In December, South Korea seized a Hong Kong-registered ship that allegedly transferred oilto a North Korean vessel in international waters. That same month Reuters reported on Russian tankers supplying fuel to North Korea by transferring cargoes at sea, though there was no evidence of Moscow’s involvement.

North Korea is known to sell arms to various nations. In August 2016, custom agents in Egypt discovered more than 30,000 rocket-propelled grenades under bins of iron ore (paywall) in a ship originating from North Korea but flying a Cambodian flag. And North Korea is believed to have profited handsomely by selling arms to the regime of Bashar al-Assad during the Syrian civil war.

Intercepting ships is only one thing the officials gathering in Vancouver will discuss to punish Pyongyang for its nuclear program. North Korea is well aware the idea is under consideration, so expect more fiery rhetoric about the “merciless self-defensive” measures it will take if hit with a naval blockade.