NEW YORK (CNNMoney.com) — Falling stock markets around the globe and the credit crunch are putting the pension funds of some of the largest U.S. companies into deeper financial holes, according to a report released Monday.

Since the credit crunch hit last fall, pension plans funded by S&P 1500 companies have lost about $280 billion in assets, according to an actuary at Mercer, a human resources consulting firm.

On paper, the losses from last October tally $160 billion. However, according to Mercer actuary Adrian Hartshorn, the asset losses are closer to $280 billion when pension plan assets and liabilities are considered together. The losses amount to about 7% of a total $4 trillion in pension plan assets.

Companies should be concerned, he said, because - assuming no change in the market - a typical U.S. company can expect their pension expenses to increase between 20% and 30% in 2009. That’s due to the higher cost of servicing the pension plan’s debt and the smaller return from the plan’s assets.

“I think it’s important…