Ukraine said on Saturday 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, a statement Moscow dismissed as a "fairy tale".

Ukraine has made several similar statements about Russian aggression during months of conflict with separatists on its eastern border with Russia that it says are backed by Moscow none of which have been independently verifiable.

A senior aide to Ukrainian president Petro Poroshenko said a large 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 immediately clear what convoy Poroshenko's aide was referring to.

Russia's Defence Ministry said on Friday it had finished military exercises in southern Russia, near the Ukrainian border, which the United States had criticized 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 his 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.

"As of now, the danger of provocation has been removed, but operational staff continue to work," Chaly said.

Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, dismissed the statement by Chaly as untrue.

"Each time Kiev is more and more inventive in creating fairy tales," she said, noting that special protocols had to be completed before Russian troops could be sent abroad.

"The (Ukrainian) National Guard probably have to report about their achievements in the field, so they pretended they have some," she said.

Dmitry Peskov, the spokesman for Russian President Vladimir Putin, said by phone: "We don't know what (the Ukrainians) are talking about because nothing like that happened."

However, Ukraine's Foreign Ministry warned the danger was not over.

"Against a background of numerous violations by Russia of the border regime with Ukraine and deliveries of weapons, equipment and mercenaries, Ukraine has enough grounds for believing that this convoy can be used again for further escalation of tension," the ministry said in a statement.

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 this week on Russia to pull its troops back from the Ukrainian border and warned further intervention in Ukraine would mean greater isolation from the rest of the world.

The head of the U.S.-led alliance said Russia had massed about 20,000 troops near Ukrainian border, very close to the regions where Ukrainian government forces are fighting pro-Russian rebels.

Ukrainian officials say that frequent Russian military exercises near the border complicate the situation, and on Saturday a Ukrainian military spokesman said that the move was not a "stage in de-escalation of the situation near the border".

Ukraine and its Western allies accuse Russia of orchestrating the revolt and arming the rebels, who have declared independent "people's republics" in the two main industrial regions. Moscow denies involvement.

(Reporting by Pavel Polityuk in Kiev; Additional reporting by Gabriela Baczynska, Polina Devitt, Alexei Anishchuk and Jason Bush in Moscow and Richard Balmforth in Kiev; Editing by Louise Ireland)

Ukraine says stops Russian troops invading, Moscow calls statement 'fairy tale'