Ukraine says it will block a convoy from Russia that Moscow said contains humanitarian aid.
Officials in Ukraine and other Western nations have serious concerns about the convoy, which includes up to 280 trucks.
Some believe it could be the precursor to a Russian invasion.
"We see the Russians developing the 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 such illegal military operations in Ukraine," NATO Secretary-General Anders Fogh Rasmussen said.
The controversy over the convoy has again increased the tensions between Russia and Ukraine and raised the possibility of more military fighting.