Surging prices of coal are working against international efforts to sanction North Korea over its missile and nuclear programs.

Coal is one of major products North Korea exports to China, accounting for over 40 percent of its total exports, and the rising prices are boosting funds for the North's weapons program. Trade volume in money terms between the two countries increased 3.4 percent on-year in the third quarter this year.

"North Korean coal prices have surged by 68 percent in value recently... The average price of its anthracite coal arriving at major Chinese ports rose to US$99 a ton in recent weeks from around $59 in early August," the Wall Street Journal reported Tuesday.

"In August, North Korea exported 2.47 million tons of coal to China, the highest monthly total on record. Exports slipped in September but remained higher than a year earlier."

The main reason is new mining restrictions in China to tackle overcapacity and supply disruptions after heavy rains there in July. Global coal supply has also dwindled because of shutdowns at two major coal mines in Australia.