Formosa operates a 540,000 barrels-per-day (bpd) refinery at Mailiao, making that one of Asia's ten largest refineries by capacity.

Weak gasoline margins have prompted the refiner to trim gasoline supplies this month and this would affect its overall average refining throughput as a result, Lin said.

Formosa, Asia's top naphtha importer, also operates three naphtha crackers which have a total capacity of 2.93 million tonnes per year of ethylene, a building block for plastics.

But it reduced the average throughput of the naphtha crackers to about 95% of total capacity in June because of weaker demand for ethylene caused by maintenance at downstream units, he said.

"The plan is to restore the cracker runs to 100% this month but this is subjected to downstream demand," said he said.

(Reporting by Seng Li Peng; editing by Christian Schmollinger)