Moscow has denied claims by Nato and the Ukraine government that it has sent military units into the country.
The denial from Russian's defence ministry comes as Nato's top military commander, US. Air Force General Philip Breedlove, appears to support claims from Kiev about a Russian military operation underway in the east of Ukraine.
We have seen columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops entering into Ukraine.
Earlier today, Reuters reported that Ukrainian government forces are redeploying in preparation for a possible new offensive by pro-Russian separatists in eastern regions and that columns of Russian tanks had crossed the border overnight.
The eastern Ukrainian city of Donetsk was under heavy bombardment overnight, the Organisation for Security and Co-operation in Europe said.
The worst shelling in weeks lasted over eight hours.
Ukraine's military accused Russia on Friday of sending a column of 32 tanks and truckloads of troops into the country's east to support pro-Russian separatists fighting government forces.
Russia has denied arming pro-Russian rebels in eastern Ukraine who have broken away from the Kiev government.
A column of 32 tanks has entered Ukraine from Russia, according to the AFP news agency.
The agency reported the Ukrainian military as saying that after the latest fighting left five dead and at least 31 injured.
Military spokesman Andriy Lysenko said a convoy made up of 32 tanks, 16 howitzer cannons and 30 trucks of troops and equipment crossed the border into the rebel-controlled Lugansk region on Thursday.
Another column including trucks and three mobile radar stations crossed another border point in the same region, AFP cited him as saying.
Ukraine's President Petro Poroshenko has called elections being held in the rebel-held east of the country illegitimate.
He said that Ukraine and the rest of the world would reject the results, and called on Russia to do the same.
Pro-Russian separatists will vote to set up a breakaway regional leadership in the eastern Ukraine region of Donetsk and Luhansk aiming to take their war-torn region closer to Russia and defying Kiev and the West as the big guns still boom across the territory.
FIFA president Sepp Blatter has confirmed that Russia and Ukraine will be kept apart in the 2018 World Cup group stages should the Ukrainians qualify for the tournament.
Political tension between the neighbouring states continues to be high with Kiev accusing Moscow of backing Russian separatists in the east of Ukraine.
Blatter replied "you can be sure about this" when asked whether hosts Russia would be kept apart from Ukraine following a politically driven outbreak of violence during a recent match between Serbia and Albania.
Despite accepting the two nations should kept apart, Blatter has refused to support claims the tournament should not be played in Russia if the political climate is still volatile in 2018.
“A boycott never achieves anything and does not have a positive effect. FIFA is fully supporting the World Cup in Russia,” he said.
Ukrainians vote in an election that is likely to install a pro-Western parliament and strengthen President Petro Poroshenko's mandate to end separatist conflict in the east, but could fuel tension with Russia.
People wrapped up warmly on a cold, clear day to vote in the first parliamentary poll since protests in the capital Kiev last winter forced Moscow-backed leader Viktor Yanukovich to flee and ushered in a pro-Europe leadership under Poroshenko.
In eastern regions controlled by the army, soldiers armed with automatic rifles and wearing bulletproof jackets guarded polling stations under the yellow and blue Ukrainian flag.
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The International Committee of the Red Cross (ICRC) confirmed that a Swiss staff member was killed by a shell that landed close to its office in Donetsk, eastern Ukraine.
"One of our colleagues, a Swiss national, was killed in Donetsk, Ukraine, when a shell hit close to our office. We're deeply distressed by this loss," ICRC spokesman Ewan Watson told Reuters.