Pension plan funding levels among large publicly held U.S. employers fell to a new low last year as falling interest rates fueled a rise in the value of plan liabilities, which more than offset strong investment results and hefty employer contributions, according to a Milliman Inc. survey released Monday.

Defined benefit plans offered by the 100 U.S. employers with the largest pension programs were, on average, 77.2% funded at year-end 2012, down from 79.2% funded in 2011, which was the previous lowest funding level, and 83.9% funded in 2010.

In all, the market value of pension plan assets, aided by average investment returns of 11.7%, increased about $73 billion to about $1.319 trillion in 2012.

However, the value of plan liabilities, inflated by low interest rates, leaped by $134 billion to $1.707 trillion.

The nearly $389 billion funding deficit was the largest in the 13-year history of the Milliman survey.

“People are probably getting tired of hearing me, but pension funding status will continue to be tied to interest rates. If rates stay low — and all indications are that they will through 2014 — these pension plans will struggle to fill their funding gap,” John Ehrhardt, a Milliman consulting actuary in New York and co-author of the pension study, said in a statement.

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But some companies took steps to pare back the size of their pension plans through “de-risking” strategies, such as offering certain pension plan participants the option to convert their annuity to a cash lump sum benefit or shifting plan liabilities to insurers through the purchase of group annuities. In all, at least 15 of the 100 surveyed companies engaged in significant pension de-risking efforts in 2012, according to Milliman.

The de-risking approaches taken in 2012 by Ford Motor Co., General Motors Co. andVerizon Communications Inc. alone reduced those companies' benefit obligations by a whopping $41 billion, according to Milliman.

In fact, GM's de-rising strategy, in which the automaker used billions of dollars in plan assets to purchase a group annuity from Prudential Insurance Co. of America, as well as to fund lump sum benefit offers made to tens of thousands of plan participants, resulted in GM losing its position as the largest — measured by amount of plan assets — private employer pension plan sponsor in 2012, according to the Milliman survey. GM's pension plans, though retained its rank as having the largest liabilities of any plan sponsor: $111.4 billion.

Among surveyed employers, Armonk, N.Y.-based International Business Machines Corp. had the largest defined benefit program with $91.7 billion in assets. Its plans were 86.4% funded last year, down from 89.3% in 2011.

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GM had the second-largest pension program with $83.6 billion in assets, a sharp decline from $108.9 billion in 2011. Benefit obligations also plunged dropping to $111.4 billion in 2012, from $134.3 billion in 2011. GM's funded ratio dropped to 75.1% in 2012, compared to 81.1% in 2011.

Nextera Energy Inc., a Juno Beach, Fla.-based energy and utility company, had the highest funded ratio in 2012 at 142.7%, down from 147.1% in 2011.

Atlanta-based Delta Air Lines Inc. had the lowest funded ratio — 38.1% in 2012 — down from 40.4% in 2011.

Just six employers had plans that were at least 100% funded. In last year's survey, Milliman found that 10 employers sponsored pension programs that were at least 100% funded.

On the other hand, 18 employers sponsored pension plans that were less than 70% funded last year, compared to 15 2011.

Employer plan contributions hit a record $61.5 billion last year, up from $55.1 billion in 2011.

In conducting the analysis, Seattle-based Milliman analyzed financial reports of publicly held companies sponsoring the 100 largest pension programs for which full-year data was available.