North Korea doubled its exports of an alloy and carbon additives used in steelmaking to China in September, official customs data shows, while Pyongyang ramped up imports of fuel from Beijing.

China’s General Administration of Customs (GAC) released a detailed breakdown of traded goods on Tuesday, after reporting Sunday that China-DPRK trade reached nearly $300 million last month.

GAC data shows that North Korea supplied China with nearly $3.7 million worth of metal goods falling under HS Codes 72, 82 and 83, which are banned from export under U.N. Security Council Resolution (UNSCR) 2397.

Ferrosilicon, an alloy