BEIJING, June 30 -- China's quarantine watchdog has returned 1.25 million tonnes of corn shipments from the United States in recent months over genetic modification concerns, an official said on Monday.

Since October, Chinese authorities have been finding U.S. corn shipments tainted with genetically modified (GM) strain MIR162, said Lu Chunming, an official with the General Administration of Quality Supervision, Inspection and Quarantine.

MIR162, a kind of insect-resistant transgenic corn, is not approved by China's agriculture ministry.

The first batch of MIR162-tainted corn was found in Shenzhen in south China's Guangdong Province, and more have been detected at other ports.

The country rejected 601,000 tonnes of unapproved GM corn imports in 2013, according to the administration.