The resolution is vague on how to implement sanctions on North Korean air cargo, in contrast to the provisions for sea cargo.[1] However, it is alleged that North Korea uses air traffic more than sea traffic to transport or exchange weapons technology.[45] The Congressional Research Service report identifies the Pyongyang–Tehran air route as a matter of concern, as most of the US$1.5 billion North Korea earns in weapons sales comes from Iran.[46][47] In addition to this, the report claims the air route is also used for the exchange and collaboration of WMD technology, as well as a route for visits by scientists, technicians and nuclear and ballistics officials.[25] Most of the route passes over China, and a U.S. delegation held talks with Chinese officials over how to implement the resolution regarding the aircraft which regularly refuel at Chinese airports,[48] but did not report on the response.[6]