(Updates prices, changes dateline to London)

By Polina Devitt

LONDON, Aug 6 (Reuters) - Copper prices hit their highest in

six months on Thursday as worries about tightening stocks

outside the United States coincided with concerns about reduced

supply of concentrates from the Democratic Republic of Congo.

Benchmark three-month copper on the London Metal

Exchange was 1.2% higher at $14,275.50 a metric ton by 1005 GMT

after hitting $14,369.5, the highest since the January 29 record

of $14,527.5.

Congo banned exports of copper concentrate and cobalt

concentrate as it tries to force domestic processing and retain

more value from its mineral resources, according to a new

government order reviewed by Reuters on Thursday.

The development inflated worries about supply and added to

uncertainty over possible U.S. import tariffs, which kept COMEX

copper above the LME benchmark and drew metal into U.S.

warehouses.

COMEX copper futures hit a record high of $6.723 per lb

on Thursday and traded at a premium against the LME contracts

as the market continued to price in a potential

U.S. import tariff decision.

The most active COMEX September contract was last up

1.3% at $6.817 per lb ($15,029 a ton).

The premium for LME cash copper over the three-month

contract widened to $130 a ton, the most since

October, signalling tightening conditions for near-term supply

with total copper stocks in the LME-registered warehouses at

226,650 tons, the lowest since mid-February.

StoneX estimated at least 1.2 million tons of copper have

entered the U.S. since February 2025, leaving about 64% of

globally visible inventories in the country.

In other LME metals, nickel was down 2.2% at $16,735

a ton, having touched its lowest since July 13 at $16,565 after

Chinese market information hub SMM reported that one of

Indonesia's nickel miners had received an additional quota for

nickel ore production.

LME aluminium rose 0.4% to $3,253.50, zinc

added 1.7% to $3,790, lead gained 0.5% to $1,894.50 and

tin edged up 0.1% to $56,515.