At least four people are reported to have been killed in an ongoing attack by armed men on a military base in the Pakistani city of Karachi.
Some of the attackers are still inside the base and commandos and marines are continuing to battle them several hours after the assault began, 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. Two maritime patrol aircraft were destroyed and at least nine people were also wounded in the assault.
He said the navy is in "a high state of alert" while "the attack raises a lot of questions as to how such a large number of people, so well armed, could have actually gotten into the base".
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.
"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.
"One of our officers and one Navy personnel have been martyred," Commodore Irfan ul Haq, a spokesman for the Pakistan Navy, told the AFP news agency.
Five explosions were heard at the base ahead of the attack, according to Pakistani Express TV.
Yusuf Raza Gilani, Pakistan's prime minister, condemned the attack.
"Such a cowardly act of terror could not deter the commitment of the government and people of Pakistan to fight terrorism," Gilani said in statement.
No group has claimed responsibility for the attack, but al-Qaeda-linked Taliban fighters have vowed to avenge the killing of Osama bin Laden, the al-Qaeda leader, by US special forces.
They have carried out several attacks since his death on May 2.
The BBC's Shoaib Hasan in Karachi says that there are hostages inside the Gunmen have attacked a military base in the Pakistani city of Karachi, killing at least 11 soldiers, officials say.
The well-armed attackers set off explosives and have been fighting gunbattles with navy personnel at the Mehran naval aviation base.
The gunmen are now holding hostages, including Chinese military personnel.
No group has claimed the raid, but the Pakistani Taliban have vowed to avenge the killing of Osama Bin Laden by US special forces on 2 May.
They have carried out several attacks since then.
On Sunday militants stormed three hangars housing aircraft at the Mehran base, according to officials.
Interior Minister Rehman Malik said: "We have been able to confine them to one building and an operation is underway either to kill or capture them."
Syed Shoaib Hasan
BBC News, Karachi
Clouds of black smoke billowed over the Mehran airbase when I arrived about half an hour after the attack began.
Ambulances with sirens wailing stood outside the gate of the compound, which was sealed soon after the attack began. They were joined by dozens of TV satellite trucks and journalists.
The base is located next to the Shahrah-e-Faisal - Karachi's main road - and many interested onlookers stopped their vehicles to find out what was happening.
In the distance, flames could be seen in the horizon beyond the perimeter wall.
Initially, there was intermittent gunfire. However, 15 minutes after I arrived on the scene there was an exchange of fire that lasted for several minutes.
We had to lie down on the ground, as the bullets and shrapnel whizzed over our heads.
Later, as I left the scene, several army trucks carrying dozens of army troops and armoured personnel carriers hurried into the compound to boost the troops battling the militants inside.
Flames can be seen in the distance, and intermittent gunfire continues as troops battle the militants inside, says the BBC's Syed Shoaib Hasan at the scene.
Their first targets were aircraft parked on the tarmac and equipment in nearby hangers.
Eyewitnesses say the militants used rocket propelled grenades to damage and destroy several warplanes. These included the Pakistan navy's premier anti-submarine attack jet - the US made P-3C Orion.
At least two of these multi-million dollar aircraft were set ablaze.
The gunmen then opened indiscriminate fire, killing several naval personnel as they carried their deadly raid into the heart of the base.
Subsequently, navy commandos and marines launched a counter assault. Dozens of heavily armed army reinforcements also arrived to provide cover.
Some of the militants have now been killed, officials say.
The remaining gunmen have taken several officials, including Chinese military personnel, hostage inside a building. Security officials say commandos are now being sent in to clear this area.
On Friday the Taliban bombed a US consulate convoy in Peshawar, killing one Pakistani.
Other attacks by Pakistani militants this month include a raid on a security post that killed two police in the north-west and a twin suicide bombing at a paramilitary police training centre.
Fire and smokes erupt from a Pakistan' military air base after an attack by militants in southern Pakistan's Karachi on May 23, 2011. Four foreigners reportedly killed in a terrorist attack at a Pakistan Air Force base in Karachi late Sunday night, local Urdu TV channel ARY reported. (Xinhua/Masroor)
Pakistani troops arrive at Pakistan military air base after an attack by militants in southern Pakistan's Karachi on May 23, 2011. At least four people were killed and one plane was destroyed following attacks launched by terrorists late Sunday night at a Pakistan air base in Karachi. (Xinhua/Toheed)
People gathered outside the Pakistan military air base after an attack by militants in southern Pakistan's Karachi on May 23, 2011. At least four people were killed and one plane was destroyed following attacks launched by terrorists late Sunday night at a Pakistan air base in Karachi. (Xinhua/Toheed)