US on alert as it hails success of mission to kill terrorist chief

The US and its allies stepped up security in their largest cities last night amid fears of a possible revenge attack from al-Qa'ida militants following the killing of Osama bin Laden in Pakistan as officials in Washington began openly to ask how he could have lain low there without some local network of support.

Hours after Barack Obama announced that Bin Laden had been killed, triggering spontaneous street celebrations in the US, officials warned of the possible repercussions. "Though Bin Laden is dead al-Qa'ida is not," said Leon Panetta, director of the CIA. "The terrorists almost certainly will attempt to avenge him and we must – and will – remain vigilant and resolute."

In London, David Cameron said the West would need to be "particularly vigilant" in the weeks ahead.

The warnings came after a remarkable day in which it was revealed that the man who had waged war on the West in such cruel fashion when his network struck in New York and Washington on 11 September 2001, had been tracked down to a comfortable home in Abbottabad with few outward-facing windows and no internet or telephone access, and killed after a Sunday operation that lasted 40 minutes.

While many experts had suggested the al-Qa'ida leader might have been hiding in Pakistan, it was always assumed it would have been in the wild tribal areas close to the border with Afghanistan and not in the centre of a garrison town that is home to thousands of Pakistani troops. John Brennan, the anti-terrorism adviser to President Obama, said he thought it "inconceivable" that he could have been in Abbottabad without support from within Pakistan.

His presence there is hugely embarrassing to Pakistan and while US officials were last night quoted as saying they did not believe the Pakistani authorities were aware of his location, Islamabad will struggle to convince its critics that it was innocent. It seemed that Bin Laden also made what was almost certainly his last propaganda tape shortly before being killed. While information about the tape was sketchy, it was expected to surface soon.

The operation on Sunday night, which was carried out by helicopter-borne US special forces, followed a months-long intelligence operation in which the CIA had monitored the three-storey residence around the clock. They had been alerted to it after tracking the movements of a man, long suspected of being a courier for Bin Laden.

There were differing accounts yesterday of who inside Pakistan, if anyone, had advance warning of the US operation but US Secretary of State Hillary Clinton told reporters that co-operation with the Pakistanis had helped lead the US to the high-walled compound that was stormed by Navy Seals. She declined to comment on any possible reward resulting from the killing of Bin Laden. Television pictures from inside the house showed bloodstains on the floor next to a large bed. Officials said that one of Bin Laden's wives, one of his sons and two other men were killed. Mr Brennan said a woman identified as a wife of Bin Laden had been used as a human shield to protect him and had also died. Bin Laden might have been taken alive if he had not resisted in the fire fight. In fact he reportedly pulled a gun though Mr Brennan could not say if he fired it.

Zabiullah Khan, 20, an eye-witness to the US operation said: "There were two black gunship helicopters. I couldn't see them clearly in the night, but it was obvious that they weren't Pakistani. We don't have gunship helicopters."

Another resident, Muhammad Riaz, who lives five minutes from the compound, heard one of the helicopters crashing. "We rushed out of the house immediately," he said. "When we came outside, I saw the helicopter on fire, there was smoke rising out of it."

No photographs of Bin Laden were released yesterday but officials in Washington said some were taken in the aftermath of the operation. They may be released later. The officials also revealed that DNA tests had been carried out on his body and that they showed a 100 per cent match to samples from relatives that were gathered in the aftermath of September 11.

Facial mapping technology also convinced the US counter-terrorism team that they had killed the right man. Apparently he was shot in the head.

Officials also said Bin Laden's body had been buried at sea, after being washed according to Islamic tradition. The decision to quickly dispose of the body from the deck on the USS Carl Vinson into the waters of the northern Arabian sea was taken to avoid a grave on land becoming a shrine for his followers.

Top US leaders were at pains meanwhile to underscore the significance of the demise of Bin Laden. The "world is safer, it is a better place, because of the death of Osama bin Laden," President Obama said. "Today we are reminded that as a nation there is nothing we can't do."