It's widely believed in the US, and in other Nato countries, that Russia is not only arming the rebels but sending soldiers to fight alongside them, so the pressure is increasing on the White House to ramp up military supplies to the Ukrainian government to help it resist a new offensive.
Currently the US only provides non-lethal equipment, such as gas masks, night-vision goggles and radar. How much further can it go without escalating the conflict or being seen as an aggressor?
A report by three US think-tanks this week calls for more non-lethal aid - such as drones, secure communications equipment and armoured Humvees - but also "defensive" lethal assistance, specifically anti-tank missiles.
"Ukrainian light anti-armour capabilities are severely lacking at a time when the Russians have moved large numbers of tanks and armoured personnel carriers into the Donbass," the report says.
Read the full article at BBC.