NATO allies must stay the course in Ukraine, the top US diplomat for Europe insisted on Tuesday, calling for alliance command and control centers to be set up quickly in Kiev's neighbors.

"We have to keep our security commitments to each other," US Assistant Secretary Victoria Nuland said, adding that all NATO allies must contribute to the new "spearhead force which will allow us to speed forces to troubled spots".

"And we must install command and control centers in all six frontline states as soon as possible," she said.