Two dogs are being held at a "secure location" following an attack which killed seven sheep and seriously injured another 16.
North Wales Police said it happened at night on Padeswood Road in Buckley, Flintshire, on 20 November.
In a video posted on Twitter, a police officer said he found a "scene of absolute devastation."
The injured sheep are getting veterinary treatment, the farmer said.
"It was horrible to see all the blood and mess and the animals in distress," said John Lightfoot, from Cae ap Edward farm at Llanarmon yn Ial.
The 16 badly injured sheep have been given injections he said, "and we are caring for them and hope most will pull through".
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Mr Lightfoot said he lost seven ewes in another attack at a different grazing area two months ago.
"[It] was never proved what dogs were responsible," he said. "Hopefully we will find out the owners of the dogs this time."
Two German Shepherd dogs suspected of involvement are being held at a secure location to prevent further attacks, North Wales Police said.
"Livestock attacks are extremely distressing not only for the animals, but for their keepers too," said Sgt Peter Evans from the force's rural crime team.
The costs, both financially and emotionally for those who own or find dead and injured animals, are wholly unacceptable," he added.
Mr Evans called on on owners to either keep their dogs under control, or leave them at home.
A police community support officer, Iwan Owen, said in a video on Twitter he was left feeling "sick" by the scene of the attack.
"The farmer is absolutely devastated and the sheep are suffering," he said. "We get quite hardened to this here on the rural crime scene, but today its left us feeling... sick, really awful."
Separately, Gwent Police said there were a series of a sheep attacks in the Govilon and Llanfoist areas of Monmouthshire.
One farmer had six sheep killed and 19 others injured, the force said.