WASHINGTON - The "battle to stop al-Qaida and its syndicate of terror" is not over after the killing of Osama bin Laden, US Secretary of State Hillary Clinton said on Monday.

She said the operation to find and kill bin Laden nearly a decade after the Sept 11 terrorist attacks showed the US would never abandon its pursuit of justice.

People gather in New York's Times Square on Monday in a spontaneous act of celebration after the announcement that al-Qaida leader Osama bin Laden had been killed in Pakistan. [Photo/Agencies]

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Bin Laden, the world's most-wanted man and the elusive mastermind behind the attacks on Sept 11, 2001, was killed in a firefight with elite US forces on Monday, then quickly buried at sea in a stunning finale to a furtive decade on the run.

"Justice has been done," US President Barack Obama said in a televised statement on Sunday night in Washington (Monday morning Beijing time).

Bin Laden's death drew a mix of celebration and relief from his enemies around the world, shock among his followers and warnings that his demise would not bring an end to terrorist attacks.

Jubilant crowds gathered at ground zero in New York, where the twin towers were brought down by bin Laden's hijackers 10 years ago, and outside the White House from where Obama announced bin Laden's slaying in a helicopter raid in Pakistan.

"I'm very happy that bin Laden is now dead," Lema Dero, a parking lot manager, told China Daily at Lafayette Park in front of the White House. "He killed so many people."

A small US elite force killed 54-year-old bin Laden early on Monday local time in the town of Abbottabad, about 50 km north of the Pakistani capital, Islamabad, US and Pakistani officials said. The team took custody of his remains and an American official said bin Laden had been buried at sea.

Three adult males were also killed in the raid, including one of bin Laden's sons, whom officials did not name.

The military operation took mere minutes.

"This was a kill operation," a US national security official said, making clear there was no desire to try to capture bin Laden alive in Pakistan.

"I heard a thundering sound, followed by heavy firing. Then the firing suddenly stopped. Then more thundering, then a big blast," said Mohammad Haroon Rasheed, a resident of Abbottabad after the helicopters had swooped in and then out again.

US officials said the CIA tracked bin Laden to his location, then elite troops from Navy SEAL Team Six, a top military counter-terrorism unit, flew to the hideout in four helicopters. Bin Laden was shot in the head in an ensuing firefight, the officials said, adding that he and his guards had resisted his attackers. US personnel identified him by facial recognition, an official said.

Initial DNA results show a "very confident match" to bin Laden, a US official said on Monday.

The test showed "high confirmation" that it was bin Laden that was killed in the raid in Pakistan, the official said.

Obama said neither Americans nor civilians were harmed in the operation.

He provided few details of the operation in his late Sunday night address to the nation beyond saying that he had personally ordered it be carried out. Other officials said it was so secretive that no foreign officials were informed in advance, and only a small circle inside the administration was aware of what was unfolding half a world away.

Bin Laden was buried at sea because Islamic practice and tradition call for speedy burial, a US official said, but did not say where. Finding a country willing to accept the remains of the world's most wanted terrorist would have been difficult, the official added.

The US and Pakistani officials spoke on condition of anonymity because of the sensitivity of the information.

Pakistan declared the killing of bin Laden a "major setback" to global terrorism.

"Osama bin Laden's death illustrates the resolve of the international community, including Pakistan, to fight and eliminate terrorism," the government said in a statement. "It constitutes a major setback to terrorist organizations around the world."

Dong Manyuan, an anti-terror expert at the China Institute of International Studies, told China Daily the death of bin Laden may bring a new round of reprisals against the US and pro-US forces.

The Pakistani Taliban threatened attacks against government leaders, including President Asif Ali Zardari, the Pakistan army and the US on Monday.

Obama struck a less than boastful tone in his announcement, although he said the death of bin Laden was "the most significant achievement to date in our nation's effort to defeat al-Qaida.

"His death does not mark the end of our effort. There's no doubt that al-Qaida will continue to pursue attacks against us. We must and we will remain vigilant," he added.

Moments after Obama spoke, the US State Department put embassies on alert and warned of the heightened possibility for anti-American violence. In a worldwide travel alert, the department said there was an "enhanced potential for anti-American violence".

Reuters and Li Xing in Washington contributed to this story.