American International Group Inc. (AIG - Get Report) shares were down more than 7% Thursday after the insurer reported a $622 million loss in the fourth quarter after reporting a profit a year ago. 

The loss was due mainly to losses from its catastrophe segment. The company's insurance unit reported an underwriting loss of $1.1 billion, compared to a loss of $848 million a year ago, due mainly to losses associated with Hurricane Michael and the California wildfires. 

The company reported adjusted earnings of 63 cents a share, easily topping analysts' estimates of 42 cents.

"Throughout 2018, significant foundational work was undertaken to remediate AIG's core underwriting capabilities. While many issues and challenges were uncovered, we moved quickly to reduce risk and volatility, as well as implement strategies that we believe will accelerate our progress in 2019," said CEO Brian Duperreault. 

Duperreault took over AIG in May 2017 and has pledged to lead a turnaround at the New York-based insurer. 

For the year, the company narrowed its reported loss to $6 million, or 1 cent a share, though revenue fell 7.1% year over year. 

"Our work continues to restore AIG as the leading insurance company in the world and I remain confident we are on the right path to achieve long-term, sustainable and profitable growth," Duperreault said.