U.S. copper prices extended gains on Thursday, outperforming the London benchmark as worries about US import tariff plans inflated the regional premium and concerns about economic growth for the wider market.

The most active May copper futures on the U.S. Comex exchange rose 0.8% to $4.624 per lb. Three-month copper on the London Metal Exchange lost 0.3% to $9,435 a metric ton by 1120 GMT.

"Trade discussions are adding layers of uncertainty to base metal markets," said WisdomTree commodity strategist Nitesh Shah.

China, the world's top metals consumer, on Thursday urged the U.S. to halt an investigation into potential new tariffs on copper imports, vowing to retaliate if Chinese entities become caught up in the levies.

U.S. President Donald Trump clouded the outlook for looming levies on top trading partners Canada and Mexico on Wednesday by signalling that they would take effect on April 2 instead of the previously stated deadline of March 4.

But a White House official said levies on Mexican and Canadian goods remained in effect "as of this moment", stirring further uncertainty about U.S. trade policy. Trump also floated the idea on Wednesday of a 25% "reciprocal" tariff on European cars and other goods.

The Comex copper premium over the LME contract widened to $759 a ton from $633 on Wednesday.

Meanwhile, LME aluminium rose 0.5% to $2,644.50 a ton, zinc added 1.1% to $2,840.50 and nickel gained 1.8% to $15,860 while lead lost 0.3% to $2,004.

LME tin fell 1% to $32,030 while the April contract on the Shanghai Futures Exchange slumped by 3.7%, which analysts attributed to expectations that could resume from major supplier Myanmar's Wa state.

A notice circulating on Chinese social media on Wednesday, purporting to be from the Wa State Industrial Minerals Management Bureau, outlined the procedure for securing permits for mining and exploration.

(Reporting by Polina Devitt Editing by David Goodman)