Swiss Re said on Monday that total insured losses from the storm were estimated at between 20 billion and 25 billion euros.
The group, which provides insurance for insurance companies, warned that its own exposure of $900 million "is subject to a higher than usual degree of uncertainty and may need to be subsequently adjusted."
Sandy slammed into a densely populated part of the US, resulting in "prolonged power outages, disruption to public transport and damage to other infrastructure that have made recovery efforts very difficult," a statement noted.
"It also complicates the loss assessment process," Swiss Re said.
Chief executive officer Michel Lies was quoted as saying that "Swiss Re will support our clients and partners in tackling this challenging situation, as we have done in so many instances in the past."
The so-called super storm killed more than 110 people in the US and Canada.