Quantifying our involvement in terms of an exact total number is meaningless at this stage

Saxon Riley, Lloyd's chairman

Early estimates said the atrocities may leave insurers facing bills totalling $15bn (�10.2bn).

But some experts estimate the final cost could be as high as $25bn-30bn.

Meaningless sums

Insurance market Lloyd's of London warned that any estimate of the total damage caused by the strikes was as yet difficult to estimate.

Lloyd's chairman Saxon Riley said the attacks have "generated the most complex set of insurance liabilities and interdependencies the industry has ever seen".

Any attempt to quantify Lloyds' involvement would be "meaningless at this stage", he said.

But he confirmed that Lloyds has a "substantial involvement" with the WTC as well as the two US airlines whose planes were hijacked for the attacks, American Airlines and United Airlines.

Early estimates are likely be inaccurate because damage to buildings may not come to light for months or years, a Lloyd's spokesman added.

After the 1994 California North Ridge earthquake, claims were still emerging two years later, he said.

Too hard to imagine

There are still questions over the extent of insurance cover for the building and its tenants.

Since the World Trade Center has been attacked by terrorists before, in 1993, there are reports that many tenants may have clauses in their insurance contracts specifically ruling out a terrorist strike.

And according to a report in UK newspaper the Guardian, the owners of the World Trade Center's twin towers did not have enough insurance to cover both of them.

Top insurance disasters

1: Hurricane Andrew, 1992, $19.7bn 2: Los Angeles earthquake, 1994, $16.3bn 3: Cyclone Mireille, 1991, $7.14bn 4: Storm Daria, 1990, $6.05bn 5: Storms Lothar and Martin, 1999, $6.00bn Source: Swiss Re Costs at 2000 prices

"The possibility of the loss of both structures was seen as so remote that cover was not taken out on those lines," a spokesman for the US Insurance Information Institute told the paper.

The Port Authority of New York, which built the towers and leased them to a private company earlier this year, denied the reports, insisting in a statement that its insurance was sufficient to cover the damage.

Costs to insurers

Insurers that believe they will be picking up part of the tab for the human and economic costs caused by Tuesday's attacks include Berkshire Hathaway, controlled by US investment guru Warren Buffett.

Insurers' claims estimates (in order of magnitude)

Swiss Re: $1bn Munich Re: 1bn euros ($900m) AIG: $500m Allianz: 700m euros ($636m) Zurich Financial: $400m Axa: $300m-400m Scor: $150m-200m Chubb: $100m-200m CGNU �35m ($51m) Berkshire Hathaway: 3-5% of total loss Lloyd's: 'substantial'

Berkshire Hathaway, which owns General Re, the largest US reinsurer, said it expects to incur between 3% and 5% of the industry's losses.

Fellow US insurance giant AIG later estimated its losses at $500m, and Chubb said the attacks will mean a pre-tax cost of $100m-200m from its exposure to property insurance alone.

International insurers have also been hit.

German group Allianz said that the attacks will "significantly" hurt its full year profits, forecasting claims of 700m euros ($633m), while France's Axa forecast liabilities of $300m-400m.

UK-based group CGNU said a review of its worldwide liabilities has led to an initial estimate of �35m in costs.

The American Council of Life Insurers, said that - while it does not yet know how many policyholders have died - it expects "a large volume of claims" and is preparing to pay them out swiftly.

The council's members account for three-quarters of the US insurance market.

Some weaker reinsurance firms may be put out of business by the level of claims following the attacks, analysts told the Financial Times.