(Bloomberg) -- U.S. public pension-fund assets grew by 4.2 percent in the last three months of 2013, boosted by stock market gains, the Census Bureau reported.

Assets of the 100 largest public-worker retirement plans climbed to $3.19 trillion by the end of December, up $129.9 billion from September, the bureau said yesterday. The gains pushed holdings to the highest level since the government began tracking the data in 1968.

The rising stock market is easing the strain on public pensions, which typically rely on steady investment gains of more than 7 percent to cover payments to retiring workers.

Pensions’ stock holdings climbed 5.9 percent to $1.12 trillion, the Census said. The Standard & Poor’s 500 Index of stocks climbed 10 percent during the quarter.

To contact the reporter on this story: William Selway in Washington at wselway@bloomberg.net To contact the editors responsible for this story: Stephen Merelman at smerelman@bloomberg.net Justin Blum, Mark Schoifet