MEXICO CITY (Reuters) -Grupo Mexico, a major copper producer, is looking to decide on investments for projects in the United States within the next three to five years, an executive said on a call on Wednesday, as Washington prepares to levy tariffs on some copper products.

U.S. copper prices plunged on Wednesday after an order signed by President Donald Trump fell short of the sweeping restrictions expected and left out copper input materials such as ores, concentrates and cathodes.

Grupo Mexico had earlier said it saw the proposed tariffs as an opportunity to invest some $6 billion in expanding copper projects in Arizona run by its local subsidiary Asarco, including reopening a mothballed smelting operation.

"Now with the recent developments, we're continuously evaluating if we should or we should not restart our Hayden operation," Leonardo Contreras, finance chief of Grupo Mexico's mining division told analysts in a conference call.

"We will continue to monitor on how these global changes happen on a daily basis," Contreras added.

The tariff was intended to promote domestic development but the United States depends on imports for nearly half of its refined copper needs, and homegrown projects often take years to get off the ground.

Chile, Canada and Mexico are currently its main suppliers, though China buys the bulk of their exports.

Grupo Mexico said that although the trade war between the U.S. and China could affect global economic growth and consequently demand for copper, it maintained a "very positive" outlook for long-term copper growth in Asia.