London Metal Exchange (LME) prices stayed weak through to Wednesday's close of kerb trade as a firmer dollar weighed on sentiment, traders said.
Copper closed at its lowest in a week, while aluminium was at a three-week low.
"In the absence of much else we have been selling metals on dollar strength," one LME trader said.
The dollar extended gains against major currencies after optimistic comments on the US economy from the Federal Reserve chief, reinforcing expectations of further interest rate rises.
Business was thin and investors were wary of taking on positions because of labour disputes in Mexico and South Africa.
Workers at South African Angloplat's Rustenburg nickel and copper plant said they would wait until the end of the month before deciding whether to accept or reject a bonus offer,
Anglo American Platinum last week made a provisional bonus offer of 28.8 percent, but a final figure based on an audit will be presented to workers at the end of the month.
Three-months copper ended down at $2,757.50 a tonne, from $2,784 at Tuesday's late afternoon kerb close.
"We're at crucial levels in copper, near the 100-day moving average," the first LME trader said.
Another said: "We're at a point where the market could go either way. We migh bounce off this support or we could hit a few stops and head lower."
LME aluminium fell 1.28 percent to $1,698.
"Aluminium is in much the same place as copper. This afternoon it fell below the 100-day moving average, before recovering a bit at the close," the first trader said.
Nickel fell to $14,850 from $15,100, but lead rose to $864 from $854.
"Despite the move up, lead is still pretty much range-bound," a third trader said.
However, traders said the market remained concerned about tightness in lead supplies and low stocks.
Zinc was down at $985 versus $992.50, while tin was untraded, falling to $8,750/775 against $8,850/900.