Losses are mounting for (re)insurance giants Swiss Re and AIG, as natural disasters affect third-quarter results.

Natural catastrophes will cost Swiss Re nearly $US1.1 billion ($1.55 billion) in the quarter, the reinsurer estimates.

Claims from Typhoon Jebi in Japan are expected to total $US500 million ($702.55 million), while Hurricane Florence in the US will cost $US120 million ($168.61 million).

Multiple smaller natural disasters will cost another $US50 million ($70.25 million).

Man-made disasters are expected to approach $US300 million ($421.53 million) for the quarter, with the cost spread equally across the reinsurance and corporate solutions divisions.

The Morandi bridge collapse in Italy, a shipyard fire in Germany and a dam collapse in Colombia contributed to the cost.

Cumulative losses for the year are broadly in line with expectations, Swiss Re says.

AIG reports even bigger losses. It expects a pre-tax catastrophe loss of at least $US1.5 billion ($2.11 billion), possibly reaching $US1.7 billion ($2.4 billion).

Typhoon Jebi and Hurricane Florence are largely to blame, with Typhoon Trami and revisions to loss estimates for Californian mudslides also contributing.

AIG expects catastrophe losses in Japan and Asia to be almost $US1 billion ($1.41 billion), with US natural disasters costing $US600-$US700 million ($846-$987 million).

Hurricane Michael in the US will cost $US300-$US500 million, which will be included in fourth-quarter results.

AIG estimates it has exhausted about $US700 million of the $US750 million ($1.06 billion) retention under its North America aggregate catastrophe reinsurance program.

CEO Brian Duperreault says AIG is pleased that efforts to restructure its reinsurance portfolio are mitigating exposure to catastrophe losses.