METALS-Copper slips as Trump talks down Iran truce extension

April 21 (Reuters) - Copper prices turned negative on Tuesday after U.S. President Donald Trump said he did not want to extend a rapidly expiring ceasefire with Iran and the United States seized an Iranian oil tanker in international waters.

The metal is considered a bellwether for economic health and there are fears the Middle East conflict will hurt growth.

Benchmark three-month copper CMCU3 on the London Metal Exchange was down 0.3% at $13,224 per metric ton as of 1600 GMT, after trading most of the day in a tight range between $13,200 and $13,330.

"The lack of directional move suggests that, for now, the complex is waiting for clearer signals from macro and geopolitical developments," brokerage Sucden Financial said in a note on industrial metals.

The dollar index .DYX nudged up 0.2%, making greenback-denominated metals more expensive for holders of other currencies.

Investors were also focused on Federal Reserve chair nominee Kevin Warsh's Senate hearing on Tuesday, which will test his independence from a White House that had criticised current Fed Chair Jerome Powell multiple times for failing to cut interest rates sooner.

In China, refined copper output rose to a monthly record in March, showing no cutbacks despite last year's vow by smelters to trim output by 10% to counter falling processing fees.

Zinc CMZN3 was the sole LME metal to gain ground, rising 0.9% to $3,439.50 a ton. Investors were likely hedging their short positions on zinc, used to galvanise steel, by buying futures contracts, one trader said.

Broker Marex said the overall speculative long position on zinc was the highest since the second quarter of 2024.

Elsewhere, aluminium CMAL3 fell 0.4% to $3,542, lead CMPB3 lost 0.6% to $1,961, tin CMSN3 shed 1.4% to $49,945 and nickel CMNI3 was flat at $18,235.

Nickel contracts on the Shanghai Futures Exchange 0#SNI: opened to overseas traders for the first time in Tuesday's evening session in China.