(Reuters) - American International Group Inc AIG.N, the largest commercial insurer in the United States and Canada, reported a bigger-than-expected quarterly loss, largely due to a $5.6 billion reserve charge to cover possible future claims.

The AIG logo is seen at its building in New York's financial district March 19, 2015. REUTERS/Brendan McDermid

Shares of the company, which also raised its share buyback program by up to $3.5 billion, were down 4.5 percent in after-hours trading on Tuesday.

AIG’s net loss widened to $3.04 billion, or $2.96 per share, in the fourth quarter ended Dec. 31, from $1.84 billion, or $1.50 per share, a year earlier.

Much of the loss was due to the reserve charge for long-term risks on U.S. commercial insurance policies it has already written. AIG agreed last month to pay about $10.2 billion to a unit of Warren Buffett's Berkshire Hathaway Inc BRKa.N to take on the bulk of the risk associated with those policies.

The deal with Berkshire follows a $3.6 billion increase to reserves chalked up by AIG in the last quarter of 2015 and chief executive Peter Hancock told CNBC the insurer was reducing its reliance on commercial insurance in the United States as claims, ranging from medical malpractice to workers’ compensation, rise.

“If you look at it over a ten-year horizon, ten years ago we were doing about $15 billion in revenue on this business,” Hancock said on CNBC. “Today that’s down to about $3 billion.”

AIG’s fourth quarter marks a critical midpoint in an ambitious two-year strategic plan aimed at turning the company around.

The plan, unveiled early last year, followed pressure from billionaire activist investors Carl Icahn and John Paulson in 2015 to split the company in three because of AIG’s poor performance that year.

By slimming down, AIG could shed its designation as a non-bank systematically important financial institution (SIFI), Icahn has said.

The insurer has donned the label since its near collapse in 2008 and the government bailout that followed, driving regulators to consider some non-bank companies as “too big to fail.”

TWO-YEAR PLAN

The goals for AIG’s restructuring plan include returning $25 billion to shareholders and becoming a “leaner, more profitable and focused insurer” by trimming its property and casualty business and shedding unwanted assets, among other measures.

The $3.5 billion buyback announced on Tuesday keeps AIG on track to meet that goal.

On an operating basis, AIG reported a loss of $2.72 per share in the three months ended Dec. 31 while total general operating expenses fell 9.6 percent to $2.48 billion.

The insurer is looking to cut its gross general operating expenses by $1.6 billion by year-end.

AIG’s adjusted accident year loss ratio for its commercial insurance unit - its biggest - was 78.2 percent, up from 65.6 percent a year earlier. Adjusted accident year combined ratio for the unit rose to 108.3 percent from 95.8 percent.

A ratio below 100 percent means an insurer earns more in premiums than it pays out in claims.

Before Tuesday’s close at $66.89, AIG’s shares had risen about 12 percent since the U.S. presidential election in November.