Ukraine said yesterday it had headed off an attempt by Russia to send troops into Ukraine under the guise of peacekeepers with the aim of provoking a large-scale military conflict.
Moscow dismissed Kiev's claims as a fairy tale.
Ukraine has made similar statements about Russian aggression during months of conflict with separatists on its eastern border with Russia who, it says, are backed by Moscow. No statement has been independently verifiable.
A senior aide to Ukrainian president Petro Poroshenko said a Russian military convoy had been heading for the border on Friday under a supposed agreement with the Red Cross, but had stopped after an appeal by Kiev to Russia.
It was not clear what convoy Poroshenko's aide was referring to. Russia's defence ministry said on Friday it had finished exercises in southern Russia, near the Ukrainian border, which the United States had criticised as provocative.
"A huge military convoy accompanied by Russian soldiers and equipment was moving towards the Ukrainian border, allegedly by agreement with the Red Cross," Valery Chaly, deputy head of Poroshenko's administration, said.
No one at the Red Cross was immediately available to comment.
"A humanitarian column with 'peacekeepers' was to enter the territory of Ukraine, clearly to provoke a full-scale conflict," he said, according to Ukraine's presidential press service.
Chaly said Poroshenko held urgent talks with security chiefs and world leaders, though he did not specify which ones. Foreign minister Pavlo Klimkin said separately he had called his Russian counterpart Sergei Lavrov who had assured him the convoy would be stopped.
A spokeswoman for the Russian foreign ministry, Maria Zakharova, dismissed the statement by Chaly as untrue. "Each time Kiev is more and more inventive in creating fairy tales," she said, noting that protocols had to be completed before troops could be sent abroad.
Ukraine and the West see a growing danger of a Russian invasion under the guise of a peacekeeping mission. Kiev says any such mission would be perceived as direct aggression.
The head of Nato, Anders Fogh Rasmussen, called on Russia to pull troops back from the Ukrainian border and warned that further intervention would mean greater isolation from the rest of the world.