A drop in offshore natural gas production is behind the decline in sales of fossil fuels derived from federal and tribal lands, the U.S. Energy Department said.

The U.S. Energy Information Administration, the department's statistical arm, said Monday sales of fossil fuels from federal and tribal lands in fiscal year 2013 were 7 percent below the previous year's sales.

"One of the main drivers in the decline in sales of fossil fuels from federal and Indian lands is the drop in offshore natural gas production, even as total U.S. natural gas production has grown rapidly because of rising production from onshore shale resources on private lands," EIA said.

Though crude oil production from federal lands increased slightly last year, EIA said those gains were offset by declines in coal and natural gas.

EIA said the declines are part of a trend established in 2003. In the 10-year period ending in 2013, EIA said sales from federal and tribal lands have fallen 21 percent. Production from non-federal and non-tribal lands, meanwhile, has increased 34 percent during the same time frame.

Sales of fossil fuels from federal and tribal lands made up about a quarter of all fossil fuel sales in the United States last year.