United States property insurers are likely to pay out more than $20.3bn (�13.6bn) in claims over the 11 September attacks," a US industry body has estimated.

The latest figure is 22% more than the estimate reached shortly after the suicide hijacking attacks on New York and Washington.

The new figure makes the attacks the "costliest US catastrophe ever", said Insurance Services Office (ISO), an actuarial firm which is the leading claims-tallying service for US insurers.

Shortly after the attacks, it was thought that Hurricane Andrew in 1992 might continue to top the league.

More to come

ISO said it expects to raise its estimate again as more claims trickle in.

The collapse of the World Trade Centre destroyed swathes of upmarket Manhattan office space.

In Washington, the plane that crashed into the Pentagon took out a huge chunk of the building.

Insurance payouts immediately after the attacks narrowed the US trade gap for September as foreign insurers paid massive sums to American companies.

Overall, analysts expect the final tally from the disaster to be closer to $40bn.

Total to be double

The ISO's figure of $20bn includes claims for property damage and pay outs to firms for interruption to business, but there are many other kinds of claims.

Life cover, health care insurance for those injured, workers' compensation and aviation insurance claims are not included in the ISO figure.

Shortly after the tragedy, ISO put a figure of $16.6bn on the likely cost of property insurance claims.

Part of the reason for the higher figure was that rescue efforts meant loss adjusters were unable to get access to the sites of the tragedies, ISO said.

It reported US insurers have received 49,000 property claims over the attacks on New York, of which 30,000 were personal claims, 15,000 related to commercial property and 4,000 were for destroyed vehicles.

The attack on the Pentagon led to 1,500 personal claims, 200 commercial claims and 300 auto claims.

Hurricane Andrew in 1992 led to pay outs of $19.6bn.