BBC News, citing Pakistani media, reports that the airport's staff are being evacuated, and that the terminal, typically used for cargo and VIP flights, has been closed. All incoming flights have been diverted. From the BBC:
Geo News said two attackers were also reported dead and that smoke is billowing from the terminal.
The terminal is not normally used for commercial flights.
Senior police official Rao Muhammad Anwar told Agence France-Presse: "Exchange of fire is continuing. We don't know the exact number of the attackers."
UPDATE 8:44 pm: According to airport officials, the attack by militants has ended. The number of people killed, however, remains disputed. Some outlets, including the Associated Press, are reporting 13 dead while CNN, citing hospital officials, is reporting as many as 21.