A police operation is underway after a trespasser was seen running on the runway of an Adelaide airport.

A man in dark clothing was spotted running around the landing strip at Parafield Airport, with worried pilots contacting police about 3.25pm on Sunday.

The man ran into a stormwater drain before tactical police, Special Task and Rescue (STAR) Group officers and firefighters sealed off the stormwater system.

He was unable to be found, with searches continuing into Monday morning.

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The airport is used for training new pilots, with police saying no flights were affected or any aircraft put in danger.

Anyone who may have witnessed the incident or has information is asked to call Crime Stoppers.