Russian mercenaries deploy to eastern Ukraine - sources

FILE PHOTO: Alexander Borodai the Prime Minister of the self proclaimed 'Donetsk People's Republic' attends a news conference in Donetsk, June 21, 2014. Pro-Russian separatists attacked Ukrainian posts on the border with Russia and a military base, and tried to storm an airforce base overnight into Saturday, government forces said, putting a Ukrainian unilateral ceasefire under pressure. REUTERS/Shamil Zhumatov

MOSCOW (Reuters) - Russian mercenaries have deployed to separatist-controlled eastern Ukraine in recent weeks to bolster defences against Ukrainian government forces as tensions between Moscow and the West rise, four sources have told Reuters.

In recent weeks, Russia has moved tens of thousands of regular troops to staging posts closer to Ukraine and followed up by demanding urgent security guarantees from the West designed, Moscow says, to prevent Ukraine and other neighbouring countries being used as a base to attack it.