LONDON - Copper prices fell to their lowest level in more than two weeks on Thursday after a sharp rise in inventories and weak imports by top metals consumer China highlighted concerns about lacklustre demand.

Three-month copper on the London Metal Exchange (LME) fell 1.2% to $8,274 per metric ton by 1114 GMT after touching the lowest since Aug. 21.

Prices extended losses after LME data showed copper inventories jumped by 21% to 133,850 tonnes, the highest since October last year, highlighting worries about a surplus of material.

LME copper stocks have more than doubled since mid-July.

The big rise in LME stocks came as a surprise to traders since large deliveries usually occur close to the expiry of the three-month contract, which take place next week, Al Munro at broker Marex said in a note.

Import data from China, the world's biggest copper consumer, also showed signs of weakness. China bought 5% less copper in August from a year earlier, customs data on Thursday showed.

"We are cautious on the short term outlook for copper. China remains the key source of caution," said analyst Ewa Manthey at ING.

"Risks remain to the downside heading into the year end on China’s uncertain outlook."

Also weighing on the market was a strong dollar index , making commodities priced in the U.S. currency more expensive for buyers using other currencies, including in China.

Dollar strength is expected to be in place for the rest of this year, a Reuters poll showed.

The most-traded October copper contract on the Shanghai Futures Exchange (SHFE) dropped as much as 0.8% to 68,950 yuan ($9,410.79) a ton, the lowest since Aug. 29.

Yangshan copper premium fell from its highest since December last year on Tuesday at $61.50, indicating easing demand to import copper into China.

LME aluminium dipped 0.7% to $2,179 a ton, nickel dropped 0.8% to $20,430, lead edged up 0.2% at $2,229 and tin fell 1.4% to $25,910.

($1 = 7.3267 yuan)

(Reporting by Julian Luk in London; Editing by Chizu Nomiyama)