Allianz increased its estimate of losses from the US terrorist attacks by 50% to Euro1.5bn (£920m).
It was the second increase by the German insurance group.
It originally estimated claims from the 11 September attacks would result in net costs of Euro700m (£430m).
The company's share price fell by about 3%.
But the company also announced a lower than expected loss of Euro46.3m (£28.4m) for the third quarter of the year.
The loss surprised analysts who had predicted a loss arising from the company's new acquisition, Dresdner Bank.
Dresdner contributed a profit of Euro313.8m (£192.7m) along with the group's property and casualty business.
Within the property and casualty business, the group's operations in Great Britain generated gross written premiums of Euro1.898bn (£1.165.4bn) in the first three quarters of the year.
The combined ratio improved from 116.9% at the end of December to 106% on 30 September, a change put down to improvements in motor insurance.
The group is sticking to an earlier full year net profit forecast of Euro1.7bn (£1.04bn). It expects premium income to increase by about 7%.
It said premium income for the first nine months of the year rose by 8% to Euro54.9bn (£33.7bn).
