Presenting its annual review to reporters in London, Swiss Re noted that the average number of hurricanes striking the US mainland annually had nearly doubled during the past decade compared with between 1970 and 1994, but did not directly link the increase to man-made global warming.

Referring to the hurricanes in late 2005, Swiss Re said:

"While the industry dealt in a robust way with the largest insured loss ever, there are significant lessons to be learnt, if the industry is to continue to meet the considerable and growing demand for catastrophe cover." Losses to insurers as a result of hurricanes Katrina, Rita and Wilma between August and October have been estimated by the industry at between 40 and 50 billion dollars (33 and 41.5 billion euros).

Michel Lies, Swiss Re's head of client markets, said insurers needed to "produce new models for risk pricing and assessing capital requirements, which more accurately reflect the true magnitude of risk being underwritten".

He added that insurers should move to use exposure, rather than premiums, as a reference for risk. This could prevent a wave of downgrades by rating agencies as seen in the wake of Katrina.

Swiss Re produced a graph that showed an average of 2.2 hurricanes made US landfall between 1995 and this year. This compared with an average of 1.2 in the 24 years to 1994, when the storms were also less intense.

The reinsurer said the higher number of hurricanes "reflected increased surface temperatures in the North Atlantic".

Between 1943 and 1969, an average of 2.0 hurricanes struck the United States each year, Swiss Re said.

Experts have noted that other phenomena, such as cooler air at high altitudes, is needed to produce global warming.

Swiss Re's review comes after Lloyd's, the world's biggest insurance underwriting market, last month raised its estimated pre-tax loss from Hurricane Katrina to 1.9 billion pounds (2.8 billion euros, 3.3 billion dollars) from 1.4 billion.

Lloyd's also forecast that Hurricane Rita would cost it 535 million pounds and that Hurricane Wilma would add 483 million to the total.

Hurricane Katrina, which devastated the city of New Orleans on August 29, was the costliest US hurricane on record. It was followed by Rita in September and Wilma in October.

Lies, who is also an executive board member of Swiss Re, said losses from the natural disasters, which were spread broadly amongst the world's big insurance firms, exceeded forecasts owing in part to an underestimation of corporations' exposure to storm and flood losses.

Despite the earnings reduction, "the fact that the insurance industry survived an event like this needs to be underlined", he noted.

According to Swiss Re, about 50 percent of losses are to be borne by US companies, while few companies lost more than their earnings.

Lies meanwhile said that losses to Swiss Re from the British fuel depot explosions "should not be material".

Fires broke out again at the key oil depot outside London on Wednesday, as a tanker driver was quizzed over suspicions he unintentionally started the biggest industrial fire in peacetime Europe.

Orange flames and thick black smoke still filled the morning sky over the Buncefield fuel depot near Hemel Hempstead, 25 miles (40 kilometres) northwest of London after the first explosions on Sunday.

Swiss Re last month overtook Munich Re as the world's biggest reinsurer following its purchase of US peer GE Insurance Solutions for 6.8 billion dollars.

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