* LME/ShFE arb: bit.ly/2wZSAE
* GRAPHIC-2018 asset returns: tmsnrt.rs/2jvdmXl (Adds closing prices)
By Pratima Desai
LONDON, June 19 (Reuters) - Copper prices hit three-week lows on Tuesday as worries about a trade war were reinforced after U.S. President Donald Trump threatened to impose a 10 percent tariff on $200 billion of Chinese goods and Beijing warned it would retaliate.
Benchmark copper on the London Metal Exchange ended down 1.8 percent at $6,840 a tonne from an earlier $6,809, its lowest since May 31.
"A second round of U.S.-China trade tensions spur risk aversion and market concerns about the prospect of global economic growth in the future," said Xiao Fu, Head of Commodity Markets Strategy at BOCI Global Commodities.
"Meanwhile a resumption of dollar strength ... and weaker than expected Chinese economic data last week also weigh."
CHINA TARIFFS: China will impose additional tariffs of 25 percent on 659 U.S. goods in response to U.S. plans to slap duties on $50 billion of Chinese goods.
The new list is an expansion of an initial list of 106 U.S. goods published on April 4.
CHINA GROWTH: Concern about growth in the world's largest consumer of industrial metals was reflected in China's stock market and the yuan.
Shanghai stocks tumbled nearly 4 percent on Tuesday to a two-year low, while the yuan fell to a more than five-month low against the dollar.
DOLLAR: A strong dollar also weighed on commodities. Dollar-denominated metals become more expensive for non-U.S. firms when the currency rises. The dollar index against a basket of currencies hit its highest since July last year.
CHILE: Copper market participants are keeping an eye on labour negotiations at the Escondida mine in Chile.
Global miner BHP said last week it had responded to the latest contract proposal from unionised workers at its Escondida mine, the world's largest, triggering a new round of talks that could last a month or more.
"Chile's fresh employment laws introduced last year and a high copper price have emboldened union leaders," Morgan Stanley analysts said in a note.
"A tricky negotiation, but will it lead to a strike? Both sides will be keen to avoid such an outcome after wearing the high costs of 2017's stoppage."
WARRANTS: Traders are also watching a large holding, between 30 and 39 percent , of copper warrants on the LME. It has fuelled worries about a shortage on the LME market and created a premium for the cash contract over the three-month.
PRICES: Aluminium was down 2.2 percent at $2,171 a tonne, zinc fell 2.1 percent to $3,002, lead slipped 0.8 percent to $2,415, tin ceded 0.6 percent to $20,375 and nickel lost 2.1 percent to $14,655.
Additional reporting by Yilei Sun Editing by Louise Heavens and David Evans