OPEC moved on Monday to cap Nigerian oil output and called on several members to boost compliance with production cuts to help clear excessive global stocks and support flagging prices.

OPEC has agreed with several non-OPEC producers led by Russia to cut oil output by a combined 1.8 million barrels per day (bpd) from January 2017 until the end of March 2018.

OPEC states Libya and Nigeria were exempted from the limits to help their oil industries recover from years of unrest, according to Reuters.

The deal to curb output propelled crude prices above $58 a barrel in January but they have since slipped back to a $45 to $50 range as the effort to drain global inventories has taken longer than expected.

Rising output from U.S. shale producers has offset the impact of the output curbs, as has climbing production from Libya and Nigeria.