Ukraine crisis: Tanks 'cross border' from Russia
A column of 32 tanks and 30 trucks has crossed into eastern Ukraine from Russia, the Ukrainian government says.
The trucks were carrying ammunition and fighters, said a military spokesman, but the BBC cannot confirm his report.
A fragile ceasefire has been in place since 5 September, although hundreds of people have been killed since then.
More than 4,000 people have died since fighting erupted in April after pro-Russian separatists seized control in the regions of Donetsk and Luhansk.
The European OSCE monitoring mission has warned that the "bloodletting" is still going on, with numerous incidents of shelling.
Heavy artillery fire was reported in Donetsk city on Friday, according to the Interfax-Ukraine news agency.
Fears of a return to full-scale conflict were raised after the separatists held elections on Sunday in defiance of Ukraine's government and Western countries.
If the cross-border incursion is confirmed, it will be a further blow to the shaky ceasefire agreed in September, says the BBC's David Stern in Kiev.
After crossing the border in the rebel-controlled Luhansk region on Thursday, the Russian column headed for the town of Krasny Luch, said Ukraine's military.
"The deployment continues of military equipment and Russian mercenaries to the front lines," said military spokesman Andriy Lysenko.
On Friday, Russian officials responded to an earlier accusation from Canada's foreign minister that it was moving troops towards the Ukrainian border.
The defence ministry said the reports were "groundless" and made "without factual proof", according to Russia's Interfax news agency.
However, the Kremlin has yet to respond to the Ukrainian government's latest allegations of military deployments.
Nato said it was looking into the reports. In a statement, the alliance said that if confirmed the crossing "would be further evidence of Russia's aggression and direct involvement in destabilising Ukraine".
The US and Germany echoed Nato's concerns. State department spokesperson Jen Psaki said the US could not confirm the report, but that if true it would be another "blatant violation" of the ceasefire agreement.
Russia has recognised Sunday's vote which led to separatist leader Alexander Zakharchenko, 38, being sworn in as head of the self-declared Donetsk People's Republic.
Igor Plotnitsky, a 50-year-old ex-Soviet army officer, was declared head of the self-proclaimed Luhansk People's Republic, further to the east.
President Petro Poroshenko has accused the rebels of tearing up the peace deal and said that a law granting the rebel-held regions partial autonomy would be scrapped. He has ordered reinforcements to key cities in case of a rebel offensive.
But the separatists hit back on Wednesday, arguing that it was the scrapping of the special status deal that broke the peace agreement.
Although Mr Poroshenko insisted he had not given up on the peace plan, he said Ukraine had to "repel possible attacks" and not allow the "spread of this cancerous tumour".
On Thursday, the rebels accused the Ukrainian government of widespread attacks on their positions, an allegation Kiev denied.
Meanwhile, officials in Moscow said Russia backed the ceasefire and wanted further peace talks between the two sides.
Russia respected the will of voters in the separatist leadership elections, said Kremlin foreign policy adviser Yuri Ushakov, adding that he had deliberately chosen the word "respect" rather than "recognise".