Ukraine: Russian troops fighting along rebels

Ukraine accused Russian army officers on Monday of fighting alongside separatists in the east of the country and said Moscow was once more building up its troops on the joint border. A missile that downed a Ukrainian transport plane carrying eight people near the border was probably fired from Russia, Ukrainian officials said.
Background:
Ukraine says Russian army officers fighting alongside rebels [Reuters]
Ukraine conflict: Air strikes 'hit Luhansk targets' [BBC]