The Kremlin has announced that it is to send a humanitarian mission into Ukraine in cooperation with the Red Cross.
Ukraine and the West are opposed to any unilateral Russian moves into Ukraine.
The Red Cross made no immediate comment.
NATO General Secretary Anders Fogh Rassmusen highlighted concerns: “There is a high probability, we will see the Russians developing their narrative and the pretext for such an operation under the guise of a humanitarian operation and we see a military build-up that could be used to conduct illegal military operations in Ukraine.”
The West has warned any attempt to send military personal into the east of Ukraine as part of an aid convoy would be seen as an invasion.
Russian troop build up on the border with Ukraine is now a very real concern for Kyiv.
Ukraine’s Defence Council spokesperson gave the latest estimates: “As of 11am today around 45,000 armed Russian troops are now concentrated in the border areas.”
Moscow’s military moves come as Ukraine says it is poised to retake the rebel held city of Donetsk.
Rebel leader Alexsandr Zakharchenko says separatist fighters are prepared for an armed assault : “The Donetsk people’s military units are ready for a counter attack.”
Ukraine President Petro Poroshenko contacted US President Barack Obama as soon as Russia announced the humanitarian mission.
Moscow has given no indication as to when the aid convoy is due to enter Ukraine.