Shaukat Jamal, a spokesman for the Airport Security Force, said gunmen attacked the terminal late Sunday.
Pakistani television stations aired footage of what appeared to be a major fire at the airport, with the silhouette of a jet seen.
Jamal said the Pakistani military has been called in and that police are fighting the attackers.
The attack happened at a terminal not generally used for commercial flights but for special VIP flights.
Karachi is Pakistan's largest city and has been the site of frequent militant attacks in the past.
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