LONDON - Copper prices extended gains on Wednesday to the highest level in nearly four weeks on hopes that China will inject more stimulus into its economy, boosting metals demand.

Three-month copper on the London Metal Exchange (LME) added 0.2% to $8,352 a metric ton in official open-outcry trading after touching its strongest since May 11 at $8,418.

"What's driving the gains seems to be a vague hope that China's going to stimulate the economy more," said Dan Smith, head of research at Amalgamated Metal Trading.

Stimulus hopes were fanned by data that showed China's overall exports shrank much faster than expected in May while imports extended declines, presenting a grim outlook for global demand.

A state-run newspaper, however, cautioned on Wednesday that China's support measures for its beleaguered property market will need "some time" to make an impact.

Smith said that copper demand has been holding up better than many expected despite data showing China's May copper imports slid 4.6% from a year earlier.

"Copper imports are down, but it's partly because local supply has been ramping up rather than demand being particularly weak," he said.

"We're hearing in the power sector a lot of the copper wire and cable guys are getting good orders from the government to boost the rollout of the power grid."

Analysts at CITIC Futures pointed to a 70,000 metric ton supply gap in the refined copper market last month.

However, supply tightness is expected is to ease thanks to rising copper ore and concentrate stocks at ports, said analysts at Guotai Junan.

Also boosting the market was a dip in the U.S. dollar index . A weaker U.S. currency makes dollar-priced commodities more attractive for buyers using other currencies.

Among other metals, LME aluminium rose 0.1% in official activity to $2,213 a metric ton, zinc jumped 2.6% to $2,380, lead gained 0.6% to $2,038, nickel advanced 0.8% to $21,140 while tin was down 0.3% at $25,550.

(Reporting by Eric Onstad; Additional reporting by Siyi Liu and Dominique Patton in Beijing Editing by David Goodman)