"Gunmen are exchanging fire with Airport Security Force (ASF) personnel at a checkpost guarding the airport," an ASF spokesman said.
TV footage showed paramilitary vehicles and ambulances racing to the scene of the attack.
"We have suspended all flight operations at Karachi airport and we are evacuating passengers," said Mashud Tajwar, a spokesman for Pakistan International Airlines.
The checkpost is located at the entrance of an ASF camp some 500 metres (yards) away from the main airport premises.
The assault came as Pakistan launched air strikes on a militant-infested tribal district, killing 15 people in apparent retaliation for the Monday assault.