The energy sector is one of the most serious bottlenecks in the North Korean economy. Since 1990, the supply of oil, coal, and electricity declined steadily, and seriously affected all sectors of the economy. Crude oil was formerly imported by pipeline at "friendship prices" from the former Soviet Union or China, but the withdrawal of Russian concessions and the reduction of imports from China brought down annual imports from about in 1988 to less than by 1997. As the imported oil was refined for fuels for transportation and agricultural machinery, a serious cutback in oil imports caused critical problems in transportation and agriculture.