The convoy returned to Russia on Saturday, easing some of the immediate strain.
But NATO Secretary General Anders Fogh Rasmussen said the convoy crossing was especially troubling because it coincided "with a major escalation in Russian military involvement in Eastern Ukraine since mid-August, including the use of Russian forces."
A crucial moment could come Tuesday when Ukrainian President Petro Poroshenko meets with Russian President Vladimir Putin in Minsk, Belarus.
Reinforcing Europe's role, German Chancellor Angela Merkel went to Kiev on Saturday and pledged $660 million in loan guarantees to support private investment in infrastructure and schools in war-struck areas. The day before, Obama and Merkel spoke by phone about Ukraine.