MOSCOW, April 10 (Reuters) - Russian oil producer Lukoil expects its damaged CDU-6 primary unit and catalytic cracker at the NORSI oil refinery to return to operations in the second quarter following seasonal maintenance, a company source said on Wednesday.

The source also said that the damaged CDU-5 unit at the Volgograd refinery was put back into work on Feb. 21 and was now working in line with its designed capacity.

Lukoil did not immediately reply to a request for comment.

The Volgograd oil refinery, the largest in southern Russia, was attacked by a drone in early February.

The NORSI oil refinery suffered technical outages in January and was subjected to a drone attack in March. NORSI refines about 15.8 million tons of crude a year, or 5.8% of Russia's total refined crude.

Russia and Ukraine have both used drones to strike critical infrastructure, military installations and troop concentrations in their more than two-year conflict, with Kyiv hitting Russian refineries and energy facilities in recent months. (Reporting by Olesya Astakhova; writing by Vladimir Soldatkin; editing by Jason Neely)