Ukrainian soldiers have been forced to give up their defence of a strategic airport in the east of the country, in the face of what Kiev says was a Russian tank attack.It’s yet more evidence of a dramatic shift in fortunes on the battlefield, a change which Ukraine says is the result of “direct and overt aggression” by Russia, and the arrival of Russian regular soldiers to support the pro-Russian militia.Al Jazeera’s Paul Brennan reports from eastern Ukraine.