Legal constraints related to boarding vessels are the biggest challenge in enforcing sanctions on North Korea, officials from South Korea and the E.U. said at a round table on maritime sanctions enforcement, which was held in Busan on Tuesday.

“The biggest legal challenge for us is conducting an onboard inspection on high seas, even when we have this very probable grounds that this vessel has conducted illicit maritime activities,” Juyoung Lee, an official at the Export Control and Sanctions Division of South Korea’s Ministry of Foreign Affairs said.