A man is dead after a police officer shot him at St Louis Lambert International Airport, according to local police.

The shooting happened outside the doors of a terminal around 1am local time, St Louis County Police spokesperson Vera Clay said.

The rail line to Terminal 1 was closed for about three hours, with shuttles taking people from other drop-off points, but it was reopened later on Friday.

Clay said officers had noticed the man in an area where he should not have been and refused to leave.

Clay said the man showed officers a knife when they tried to get him to move.

Officers used Tasers, but the man continued to advance and one of them fired their gun, fatally wounding the man, she said.

"We don't believe this was an individual trying to catch an aeroplane or coming into town,

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