zelensky support for his country's sovereignty amid a massive build up of russian forces. >> what we know is that the aggression here is on the russian side. the military build up is on the russian side. there is a diplomatic path forward. part of the president's objective, our president's objective in having the call was to convey that clearly. >> reporter: russia has amassed 125,000 troops near ukraine's borders and in belarus to the north ukrainian security sources tell cnn. that includes tanks, ground vehicles, and aerial patrols potentially laying ground work for a quick strike and rapid reinforcement, sources say. on monday biden warned russian president vladimir putin of severe economic sanctions if russian forces invaded ukraine, steps the u.s. wasn't ready to take the last time this happened. since the russian occupation of crimea in 2014, the u.s. has given ukraine more than $2.5 billion to bolster its