BOSTON (AP) — Employee 401(k) accounts are growing fast thanks to the surging stock market and increased contributions from workers and their employers.

The average account balance grew 12 percent last year, according to Fidelity Investments. The nation's largest 401(k) administrator said Thursday that the year-end average was $77,300, up from $69,100 a year earlier. In the fourth quarter, the average balance rose 2 percent.

About two-thirds of last year's growth was attributed to investment returns and one-third to worker contributions and company matches.

The Standard & Poor's 500 stock index posted a 16 percent return last year, while a broad U.S. bond index rose 4 percent.

The average balance among the 12 million 401(k) accounts that Fidelity administers is at the highest level since it began tracking the data in 2000.