They also blew up a PC3 Orion aircraft in one of the most brazen attacks in years. Pakistani security personnel are still battling with the militants holed up at the Pakistan Air Force's Faisal airbase which also houses PNS Mehran, the naval air station, Pakistan's Interior Minister Rehman Malik said.
"I can confirm that security forces have killed six of the attackers while the rest are holed up in one building of the air station and are fighting a losing battle," Malik told Aaj television channel.
Malik said heavy contingents of special naval and military commandoes and other security forces have been rushed to the base to control the situation.
Earlier, officials of the Citizens Police Liaison Committee confirmed that at least four naval personnel were killed, while other officials said six persons were injured, including a foreigner.
A Pakistan Navy spokesman, meanwhile, confirmed that two naval officers were injured in the attack. Prime Minister Yousuf Raza Gillani reportedly spoke to the heads of all three armed forces asking them to take immediate action to control the situation.
The terrorists, numbering between 12 and 15, targeted PNS Mehran where some Chinese engineers were reportedly engaged in work within the Faisal airbase, at about 10.40 pm, sources said.
The armed men lobbed several grenades and exchanged heavy fire with security forces. The firing died out at about 11.30 pm but erupted again at midnight.
The militants apparently entered the Naval base and hangers through the Pakistan air force museum, a source said.
"They took advantage of the fact that people at that time were leaving for home from the PAF museum inside the Faisal base," the source said.
Dawn News channel quoted witnesses as saying that they had heard up to five blasts. Heavy firing continued for over 20 minutes after the first blast occurred, following which there were reports of intermittent firing.
Footage on television showed ambulances rushing to the airbase. The high-security area where the attack occurred also houses the PAF's Southern Air Command, Air War College and museum as well as PNS Mehran. Militants had also attacked two Naval buses in April in which around seven people were killed.
Hundreds of security personnel, including from the paramilitary Pakistan Rangers and elite naval and army commandos, moved into the airbase to flush out the terrorists.
Authorities used helicopters to mount surveillance over the airbase after dawn.
Heavy exchanges of fire between the attackers and terrorists continued till about 11.30 pm last night, and was then followed by intermittent firing and several blasts after midnight.
At least 10 blasts occurred within the airbase. The terrorists had been surrounded by naval and army commandos and the two sides were exchanging fire, spokesman Irfan-ul-Haq said.
The cordon around the attackers was being tightened and the operation was likely to be completed soon, he said.
The terrorists targeted several hangars where the navy's P3C Orion surveillance aircraft were parked, media reports said. Footage on television overnight showed ambulances rushing to the airbase.
Flames and thick smoke billowed into the sky from an object burning near a four-engine aircraft that looked like a P3C Orion.
No group claimed responsibility for the attack, which came 20 days after Al Qaida leader Osama bin Laden was killed in a US raid on the garrison city of Abbottabad.
The Taliban and Al Qaida have vowed to avenge bin Laden's killing by carrying out attacks in Pakistan and the US.
Image: A plume of smoke rises after an explosion at the Mehran naval aviation base which was attacked by militants in Karachi
Photograph: Reuters
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