Could you be so kind to provide some references supporting your view?

Pale of Settlements is a good start.

The brutal murders of Jews and Poles occurred during the Khmelnytsky Uprising of 1648–1657 in present-day Ukraine.[22] Modern historians give estimates of the scale of the murders by Khmelnytsky's Cossacks ranging between 40,000 and 100,000 men, women and children,[23][24] or perhaps many more.[25]

> has been Russian vs. Ukrainian fighting for nearly a 1000 years (maybe more)?

I'm still waiting for the source suggesting that ukranian and russians has been fighting for nearly 1000 years. That's what you wrote.

The borderlands suffered annual Tatar invasions. From the beginning of the 16th century until the end of the 17th century, Crimean Tatar slave raiding bands[56] took about two million slaves from Russia and Ukraine.[57] According to Orest Subtelny, "from 1450 to 1586, eighty-six Tatar raids were recorded, and from 1600 to 1647, seventy."[58] In 1688, Tatars captured a record number of 60,000 Ukrainians.[59] The Tatar raids took a heavy toll, discouraging settlement in more southerly regions where the soil was better and the growing season was longer. The last remnant of the Crimean Khanate was finally conquered by the Russian Empire in 1783.[60]

> Tatars captured a record number of 60,000 Ukrainians.[59]

How it is relevant? Your claim was "Ukranians and Russians has been fighting for over 1000 years"