Commodities are set for the biggest monthly loss since 2011 amid a price collapse that drove oil into a bear market and pushed gold to a five-year low.

The Bloomberg Commodity Index is down 9.3 percent in July, the most since September 2011, after dropping to a 13-year low this month. Oil companies such as BP Plc have started a new round of cost cutting, while shares of Freeport-McMoRan Inc., the biggest publicly traded copper producer, fell the most on a weekly basis in six years through July 24. West Texas Intermediate is heading for its biggest monthly fall this year, while copper in London is set for the worst month since January.

The index of 22 raw materials has slumped 11 percent in 2015 amid expanding gluts and concern slower economic growth in China will crimp demand. Commodities also slipped as the dollar gained on signals from Federal Reserve Chair Janet Yellen that the central bank may raise rates. The Bloomberg Dollar Spot Index is up 2.4 percent this month, the most since March.

“Bullish factors are missing for commodities in general at the moment,” Will Yun, a commodities analyst at Hyundai Futures Corp., said by phone from Seoul. “Economic growth concerns in China, a possible U.S. rate hike, a stronger dollar and the supply glut have all come into play. It seems we’ve nearly reached the bottom while the outlook for the second half for commodities doesn’t look so bright.”

Gold is set for its largest monthly drop in two years and Brent crude is heading for its third straight monthly decline. Wheat is on track for the biggest monthly drop since 2011.

“All of these commodities are priced in the dollar, so the recent dollar strength is a clear drag on the prices,” said Barnabas Gan, an economist at Singapore-based Oversea-Chinese Banking Corp. “We are expecting the Fed to initiate its rate hike this year. A firmer greenback should necessitate weaker prices across gold, crude oil and copper.”

Yellen has said the Fed is likely to tighten policy this year should the economy continue to improve in line with her expectations. Economists have put the chance of a September increase at 50 percent. A stronger dollar makes raw materials more expensive for buyers outside the U.S.