Smoke billows from inside the Pakistani naval base after the attack late on Sunday.
"At least four people are reported to have have been killed in an attack by an armed group on a military base in the Pakistani city of Karachi.
Some of the attackers were reportedly still inside the base and commandos and marines were battling them, Al Jazeera's Kamal Hyder said.
According to our correspondent, at least a dozen men attacked the Mehran base, a key naval aviation facility, late on Sunday.
"There are a number of wounded and we're getting reports that special forces have surrounded the area," he said.
The local government said that the base was under "terrorist attack".
Aamir Latif, the bureau chief of Pakistan's Online News, told Al Jazeera that a firefight was still under way as security forces attempted to wrest control of the base.
"The [fighting has been] continuing for the past 3 1/2 hours," Latif said by telephone from Karachi. "Militants have taken control of a building inside the base though it is unconfirmed if they are taking hostages there or not."
Intelligence officials said that between 15 and 20 attackers were inside the base, and had attacked three hangars housing aircraft.
A military spokesman said two Pakistan Navy staff were killed in the attack."
The attack has been going on for over five hours now more at: Al Jazeera