The first of the hijacked planes flew directly over Manhattan and hit the North Tower of the World Trade Centre at approximately 9am US time. Fifteen minutes later a second aircraft, a Boeing 737, crashed into the South Tower.

Both of the 110-storey towers of the World Trade Centre have collapsed.

The WTC complex is a symbol of America's financial power and houses a large number of financial and technology companies, including Morgan Stanley, ICAP and Cantor Fitzgerald. At the time of the attack, it is estimated up to 20,000 people would have been in the twin towers. Waters Conferences was mid-way through a three-day financial technology conference and exhibition at the Windows on the World convention centre at the very top of WTC#1.

The aircraft were hijacked at airports across the US. American Airlines confirms it has lost two planes with approximately 168 people on board: Flight 11 from Boston to Los Angeles; and Flight 77 from Washington to Los Angeles.

The attack on the Pentagon came one and a half hours after the explosions in New York.

Islamic terrorist groups are believed to be responsible for the carnage.

The entire US and Canadian aviation system has been shut down and the White House evacuated. Fighter planes are patrolling the skies over Washington in the event of further attacks.

Financial centres around the world have been put on a heightened state of alert.