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

The separatists opened fire on the bus, which was taking the men to fight just outside the rebel-held city of Donetsk, at the nearby village of Mandrykino, said a spokesman for Right Sector, extreme nationalists who are supporting government forces.

"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 told Reuters.

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 ran into the separatists unexpectedly because a government forces push against the rebels in the last few days had moved them from their usual positions, he said.

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

Video footage from the scene showed the bodies of several fighters dressed in combat fatigues lying near a bus pock-marked on the outside by bullet holes and its windows shattered.

Right Sector played a big role in Kiev street protests early this year that toppled former Moscow-backed president Viktor Yanukovich in February, setting off a confrontation with Russia and sparking the worst Russia-West crisis since the end of the Cold War.