I didn't see them with my own eyes and you didn't either. I see no reason to beleive those who claim they saw. There isn't an independent source saying the same since I don't think the Western media is one of those.
Want to know whose seen them? Ukrainians, NATO, the USA, and reporters from the AP, and reporters from the British media. They saw them cross the bloody border, multiple occasions. They've seen them shoot from the border onto Ukrainian territory. They have satellite photographs, videos taken from cell phone cameras, photographs taken by stupid Russkie soldiers boasting about shooting at Ukraine or being in Ukraine.
But, of course, they're all lying and only Soviet Russia speaks true, or so says the Putinist crowd: