Twelve Ukrainian nationalist fighters, battling a pro-Russian insurgency in eastern Ukraine, were killed early on Wednesday and an unknown number taken captive when rebels ambushed their bus, a spokesman for their group said.

The separatists opened fire on the bus, that was taking the men to fight just outside the rebel-held city of Donetsk, at the nearby village of Mandrykino, a spokesman for Right Sector, a group of ultranationalists who are supporting government forces, has told Reuters.

"They were taking part in a military operation early this morning near Donetsk and were ambushed. Twelve Right Sector fighters were killed," spokesman Artem Skoropadsky has told.

It was the group's biggest single loss of life since violence erupted in Ukraine's Russian-speaking east in April, he said.

The unit had run into separatists unexpectedly because a government forces' push against the rebels in the last few days had moved them from their usual positions.

"We have suffered terrible losses. We will avenge this," Right Sector leader Dmytro Yarosh said on his Facebook page.

(with inputs from The Moscow Times)