Police patrols are being stepped up after a spate of heating oil thefts from Flintshire churches.
At least seven churches, as well as homeowners, have been targeted in recent weeks.
John Bernard Jones, vicar at St Mary's Church, Treuddyn, and St Mary's at Nercwys, said more than 1,000 litres were stolen in three incidents.
North Wales Police has asked anyone with information to contact them on 0845 6071002, or 101 if in Wales.
Inspector Martin Best, who covers south Flintshire, said: "We are aware of all the incidents and appeal to anyone who may have any information regarding the thefts, or who may have been offered heating oil to buy in unusual circumstances, to get in touch."
Rev John Bernard Jones said security had been increased, with bars and padlocks.
He added: "We thought we had done everything we could but if they want to get at the oil, they will."
He said the incidents had always been accompanied by local people having oil stolen as well, which he said was "worrying".