Copper prices fell on Wednesday as the dollar strengthened and the slowing pace of the metal's rally raised fears of a speculative sell-off.
The dollar touched its strongest since July, making metals pricier for buyers with other currencies.
Benchmark three-month copper on the London Metal Exchange (LME) was down 1.1% at $6,702 a tonne at 1117 GMT.
Prices of the metal used in power and construction have rallied more than 50% since March as industry recovered from coronavirus closures and speculators piled into the market.
The rising dollar, signs of slowing demand in China and a loss of momentum could encourage speculators to sell positions, knocking prices lower, said Saxo Bank strategist Ole Hansen.
A fall below $6,600 would be a powerful bearish signal, he said.
"It's potentially time to sound a little bit of an alarm bell."
Factories: Euro zone manufacturing expanded faster than expected in September, but services disappointed. Japan's factory activity extended declines.
Lme Supply: A rising premium for cash copper versus the three month contract pointed to tightening nearby supply. On-warrant inventories in LME-registered warehouses are below 30,000 tonnes, from 250,000 tonnes in May. <MCU0-3> <MCUSTX-TOTAL>
China: However in China, the biggest consumer, Yangshan import premiums have halved to $55 since May and inventories in Shanghai Futures Exchange warehouses are at 193,347 tonnes from 99,971 tonnes in June. <CU-STX-SGH> <SMM-CUR-BON>
Glencore: Glencore said it would extend the life of a copper smelter and refinery in Australia, and that its Democratic Republic of Congo subsidiary Kamoto Copper Company should produce 270,000 tonnes of copper cathode in 2020.
Lead/Zinc: Oversupply of lead and zinc increased in July, data from the International Lead and Zinc Study Group (ILZSG) showed.
Aluminium: China's August imports of aluminium surged eight-fold from a year earlier as traders took advantage of lower overseas prices.
Other Metals: LME aluminium was 1% lower at $1,759.50 a tonne, zinc fell 0.9% to $2,443, nickel slipped 0.7% to $14,500, lead lost 0.6% to $1,872.50 and tin was down 0.9% at $18,085.