There's a "considerable" amount of uncertainty surrounding production from Libyan oil reserves, Marathon Oil said in a quarterly report.

Marathon left Libya out of its forecasts in its quarterly report, noting it took no oil from Libya this year. In December, the Libyan National Oil Corp. declared force majeure, absolving itself from contractual obligations because of circumstances beyond its control, at key terminals because of civil unrest.

"Considerable uncertainty remains around future timing of production and sales levels, and Marathon Oil continues to exclude production from Libya in its production forecasts," the U.S. company said in a Wednesday statement.

Libya has been unable to coordinate political efforts across the wide range of groups vying for more power since civil war ended formally in 2011.

Before NATO forces intervened in the Libyan civil war, the country was producing more than 1 million barrels of oil per day. The Organization of Petroleum Exporting Countries said some production had returned for member state Libya, with April production 53 percent above March levels to 473,000 bpd.

This week, protesters forced the closure of the Zuetina oil terminal in Libya. A spokesman for the National Oil Corp. said oil exports were shut in as a result.

Elsewhere, Marathon said improved drilling efficiency in the United States meant a net 4 percent increase in production during the first quarter.

President and Chief Executive Officer Lee Tilman said the company would keep momentum through the year through developments in Texas and Oklahoma shale basins.

"Marathon Oil is well positioned to increase activity as commodity prices and cash flows improve," he said in a statement.