Fears over personnel safety have forced Canada's Serinus Energy to call a halt to development and drilling operations in Ukraine.
The Calgary-based explorer has also shut in the Vergunskoye gas field as the area in which it is situated "is no longer controlled by the (Ukrainian) government", it said on Friday.
Production elsewhere will continue, but all drilling, workover and development operations will be wound down as they are completed or when they reach certain thresholds "at which operations can be suspended safely with no risk to the assets and people involved".
Ukraine's government continues to struggle to control large swathes of the country, particularly in the east, after Russia's recent annexation of Crimea, with pro-Russian seperatist groups looking to break away regions from the Kiev administration.
"While the company continues to produce, sell and be paid for the gas sold, it is no longer prudent to continue these active operations in a situation where the security changes daily," Serinus said.
"In particular, the area immediately in and around Lugansk where the Vergunskoye and Krutogorovskoye fields (producing 0.4 million cubic feet per day and 0.6 MMcfd respectively) are located is no longer controlled by the government and as a result production at the Vergunskoye field has been shut in.
Chief executive Tim Elliott said: "The situation in Ukraine changes daily. Notwithstanding the ceasefire declarations, the security situation on the ground remains uncertain.
"We have been operating in the field up until this point with the support and at the behest of the KUB-Gas employees who have shown tremendous bravery and determination to continue working in the face such difficult conditions.
"However, the company has decided that it is prudent to be cautious and elected to cease operations until such time as conditions in the immediate area stabilise."
Serinus said production in Ukraine is currently 34.2 MMcfd.