ISLAMABAD, Pakistan — A squad of heavily armed extremists stormed a Pakistani naval base late Sunday, leading to rocket fire and a gun battle, killing at least four naval personnel, reports and officials said.

Pakistani army and navy commandos were sent in to battle the militants. Some eight explosions were reported, including two around 1 a.m., as a fierce gun battle and fire raged at the site, a naval aviation center.

Irfan ul Haq, a spokesman for the navy, said that four naval personnel were killed and nine injured. He said that 10 to 15 terrorists were present at the site.

"We have control of the site," Haq told reporters, speaking at 2:45 a.m.

The attack began around 10:40 p.m., according to reports, with gunfire still ringing out more three hours later. There were fears that hostages had been taken.

A plane was on fire, throwing flames and thick smoke into the night sky, television pictures showed, after its fuel tank was apparently hit by a rocket.

The major terrorist assault showed once again the ability of extremists to attack even the most sensitive and well-defended security installations. Pakistani extremists, who have repeatedly shown their ability to stage commando-style "fedayeen" gun attacks, are closely allied with al Qaida.

Pakistan has been reeling from a series of revenge attacks since the killing of Osama bin Laden earlier this month. It was unclear whether the latest terrorist assault was linked to the bin Laden killing.

Local media said that some 15 militants entered the Mehran naval base, which lies off the main road that leads to Karachi airport, with several naval personnel and terrorists injured in the battle.

The U.S. Embassy denied reports that four American personnel working at the base had been killed or injured in the attack. U.S. Embassy spokesman Alberto Rodriguez said all Americans were "accounted for" and were not at the base at the time of the attack.

Ali Chishti, a Pakistani reporter at the scene, told McClatchy that around 1:30 a.m. he saw fresh groups of commandos being sent in.

"It's chaos. I can see a lot of ambulances now." said Chishti. "This fedayeen-type attack has never happened in Karachi before. It took place in such a high security zone."

In 2009, a group of armed extremists were able to enter the headquarters of the Pakistan military in Rawalpindi, causing havoc, a hostage crisis and bloodshed in an attack that lasted nearly 24 hours, which was a humiliation for the country's armed forces. The raid on the naval base Sunday may be the most serious attack on a military installation since then.

Extremists have also been able to repeatedly bomb the offices of the Inter-Services Intelligence (ISI) military spy agency.

Last month, there were three bombings against naval buses traveling through Karachi, killing eight people. The navy, seemingly not directly involved in the anti-terror fight, has been targeted less often by extremists, compared with the army, police and paramilitary forces.

"The aim of the terrorists is to create surprise and outsmart the intelligence agencies and security detail, which they managed here," said Shahzad Chaudhry, an analyst and former senior military officer. "We are lacking behind in this war. We'll have to change our methods. Business as usual cannot go on."

The Pakistani Taliban claims most terrorist attacks inside the country, though many are carried out by other allied jihadist groups.

Pakistan has been criticized for allegedly supporting some extremist groups, especially in the wake of the discovery of bin Laden living in the north of the country, but the nation has been under sustained bloody assault from Islamic militants since 2007. The army is fighting Taliban insurgents on several fronts in the north west of the country.

"Such a cowardly act of terror could not deter the commitment of the government and people of Pakistan to fight terrorism," Pakistani Prime Minister Yousuf Raza Gilani said in a statement.

Karachi is Pakistan's biggest city and its commercial capital.

(Shah is a McClatchy special correspondent.)

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