North Korea’s largest port saw a flurry of activity over the weekend, ship tracking data shows, including by vessels that are some of the country’s most prolific fuel smugglers.

According to maritime database MarineTraffic, at least 26 vessels broadcast their location within or right outside Nampho between Saturday and Sunday, a sign of continuing efforts to import oil and export coal, potentially in violation of U.N. sanctions.

While satellite imagery has long shown several dozen ships outside of the port, radio broadcasts from Nampho are usually much more sparse, as North Korean vessels