Offshore rig firm Seadrill subsidiary North Atlantic Drilling said it will be difficult to conclude an upcoming contract with Russia's Rosneft due to weak market conditions and a challenging political environment.

"As of today, it looks difficult," North Atlantic Drilling Chief Executive Alf Ragnar Lovdal told Reuters when asked how confident he was that the contract for its West Rigel rig would be concluded as scheduled.

The firm earlier said the contract was expected to start in December this year and last until November 2020.

North Atlantic Drilling received a notice of termination from Rosneft on Friday for the West Navigator vessel, which was due to start work for the Russian firm in July. The cancellation cut the firm's contract backlog by $1.0 billion.