The US is preparing to mark the 10th anniversary of the 9/11 attacks on New York, Washington and Pennsylvania. Security is tight following warnings of a possible al-Qaeda attack. The US embassy in Afghanistan has started the ceremonies, with events due in the sites where four hijacked planes struck, killing nearly 3,000 people. An official memorial to those who died is to be unveiled at the site of the World Trade Center, whose twin towers were destroyed in the attacks. President Barack Obama has said the US remains vigilant against terrorism. As in previous anniversaries, the names of all the victims will be read out at the New York event. There will be pauses for silence at the exact times when two airliners smashed into the World Trade Center's twin towers, the third was crashed into the Pentagon and the fourth was forced into a field in Shanksville, Pennsylvania, by passengers who fought with the hijackers. New York's National September 11 Memorial, to be unveiled later on Sunday, features two reflecting pools, each almost an acre in size, in the footprints of twin towers. The names of those who died on 9/11, as well as the six people killed in the 1993 World Trade Center bombing, are inscribed on the edge of the pools. In the Afghan capital, Kabul, the US flag was lowered to half-mast to remember those who died 10 years ago, as well as those who have died since. US forces were sent to Afghanistan to oust the Taleban from power after they had given sanctuary to al-Qaeda. President Obama is due to travel to all three sites for the commemorations, part of a weekend of events to mark the anniversary. Speaking on Saturday, he said the US was stronger 10 years on from the attacks. "As a resilient nation, we will carry on," he told Americans in his weekly address. "Thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al-Qaeda is on the path to defeat," President Obama said. "Yes we face a determined foe, and make no mistake - they will keep trying to hit us again. But as we are showing again this weekend, we remain vigilant," he added. Security has been tightened in New York and Washington after a warning was received that al-Qaeda may have sent attackers, some of them possibly US citizens, to bomb one of the cities. The warning, reportedly received from a CIA informant last week, was described by officials as "credible but unconfirmed". President Obama will be joined in New York by George W Bush, who was president at the time of the attacks. Speaking in Shanksville on Saturday at the unveiling of a memorial to the 40 victims of flight United 93, Mr Bush said "the United States will never forget". He lauded the passengers and crew of the flight, saying they launched "the first counter-offensive in the war on terror". In an interview with National Geographic he said the events of that day had changed his presidency dramatically. "I went from being a president that was primarily focused on domestic issues, to a wartime president. Something I never anticipated nor something I ever wanted to be." Also on Saturday, firefighters from around the world attended a memorial service in New York's St Patrick's Cathedral to honour the 343 firefighters who died on 9/11 while rescuing people from the World Trade Center towers.