CNA reported a loss of $155m (£106m), equivalent to 84c (58p) a share, in its third quarter financial results.
The Chicago-based insurer had an operating income of $156m (£107m), equivalent to 85c (58p) a share, in the comparable period of last year. It put the losses down to the estimated $468m (£321m) pre-tax costs arising from the 11 September terrorist attacks.
It has raised its estimated loss figure from a previous range of $200m (£137m), to $350m (£240m). The initial estimate was based on an assumption of industry losses totalling $30bn (£21bn) and did not include losses from general liability, individual life policies or business interruption. The updated estimate is based on assumed industry losses totalling $50bn (£34bn).
The company's estimated gross losses from the World Trade Centre attack total $1.65bn (£1.13bn).
CNA raised $1bn (£686m) during the third quarter, boosting its assets to more than $67bn (£46bn).
Chief executive Bernard Hengesbaugh said: "Despite the losses in the quarter, CNA remains a financially solid company.
"We are encouraged by support from Congress and the Bush administration for legislation to provide a federal backstop for future terrorism losses."