Oil prices are continuing to slide on fears of a slowdown in the world economy, with U.S. crude on pace to close at under $40 a barrel for the first time since the days of the global economic crisis.

Already trading around six-year lows on a prolonged slump, U.S. crude fell $1.24, or 3.1 percent, to $39.21 per barrel Monday afternoon. Earlier it dropped to $37.75.

Over the last few days investors have grown more concerned that the Chinese economy, the second-largest in the world, is slowing down. That led to a sell-off in stocks Friday and another on Monday.

Weaker growth in China, a shrinking economy in Japan, and a supply glut have punished oil prices over the last year.