(Reuters) - American International Group Inc AIG.N shares sank 4.5 percent on Friday as investors reacted to a surprise $836 million boost to the insurance giant's reserves, related to prior-year accident claims.

FILE PHOTO: A banner for American International Group Inc (AIG) hangs on the facade of the New York Stock Exchange, in New York, U.S. on October 16, 2012. REUTERS/Brendan McDermid/File Photo

AIG posted a bigger-than-expected quarterly loss on Thursday as the insurer booked huge catastrophe losses from Hurricanes Harvey, Irma and Maria. The storms coincided with AIG’s review of reserves across various U.S. and European lines, leading to the $836 million charge.

The New York-based company is one of the largest U.S. commercial insurers. Hefty losses from claims occurring in prior years have been an ongoing issue for AIG, which boosted reserves by $5.6 billion for similar reasons in February and $3.6 billion in 2015.

The development on Thursday caught analysts and investors off guard.

“It wasn’t in my earnings estimate that they were going to have that reserve charge,” said Sandler O’Neill analyst Paul Newsome.

Shares were down 4.5 percent at $62 in morning trading.

The charge also signals that AIG’s efforts in recent years to improve its underwriting were inadequate because it stems from policies the company wrote in 2016 and 2017, Newsome said.

AIG conducts quarterly reviews of reserves for its various lines. The company increased the number of those reviews during the second quarter.

“[T]his was our first chance to really view the 2016 accident year where we made many changes to our underwriting processes and tools,” AIG Chief Executive Brian Duperreault said during a call with analysts on Friday.

“The 2016 accident year is still very green, but we saw greater-than-expected claim emergence this quarter and decided to be more cautious on the 2016 and 2017 accident years,” Duperreault said.

AIG is putting changes into place to reduce the volatility of its performance, which include pursuing double-digit rate increases and bolstering reinsurance, executives told analysts on a call on Friday.

The insurer is also establishing specialized units for some businesses that are now spread throughout the company, including Lexington Insurance Company, its specialty lines insurer. The move will improve consistency and results, executives said.