Natural-gas futures plunged Thursday, falling below $4 after the U.S. government reported a larger-than-expected weekly build in U.S. inventories.

Natural gas for September delivery recently traded down 12.6 cents, or 3.1%, to $3.964 per million British thermal units on the New York Mercantile Exchange. The last time the contract dipped below $4 was April 11.

The Energy Information Administration said U.S. natural-gas stockpiles rose by 44 billion cubic feet last week, well above the 36 bcf predicted by analysts surveyed by Dow Jones Newswires.

The build was above last year's 29 bcf injection and below the five-year average build of 47 bcf.

The East region added 42 bcf and the West region added 7 bcf to storage, but the producing region had a draw of 5 bcf.

Inventories as of July 29 stood at 2.758 trillion cubic feet, 2.4% below the five-year average, and 6.3% below 2010 levels.