Karachi, Pakistan (CNN) -- Militants attacked Karachi's international airport on Sunday, killing four military personnel and leaving some passengers stuck in sealed-off areas as violence erupted, a senior Pakistani official said.
At least six militants were killed, police Inspector General Iqbal Mehmood said.
Clashes broke out after attackers entered the airport through three entrances, said Ahmad Chinoy, director-general of the citizen's police liaison committee.
All flights at the airport have been canceled, and some passengers are trapped in a lounge area, Chinoy said.
The violence started late Sunday night and continued for more than two hours into Monday morning, sending a plume of smoke over the airport.
Militants were using grenades in the attack, Chinoy said.
Maj. Gen. Asim Bajwa of the Pakistani Army said in a Twitter post that terrorists were "contained in one area" at the airport.
"All passengers in planes evacuated to safer places," he said. "Hunt for terrorists on."
The militants entered the airport through an old terminal building late Sunday, according to Sharjeel Memon, information minister for the Sindh Province.
Jinnah International Airport in Karachi is Pakistan's largest and busiest airport.