Ukraine President Petro Poroshenko claimed Russian forces had moved inside his eastern borders Thursday, just hours after the United States accused Moscow of orchestrating a new military campaign in the country.
"I made the decision to cancel a working visit to the Republic of Turkey in connection with the rapidly deteriorating situation in Donetsk region ... as Russian troops were brought into Ukraine," he said in a statement on the presidential website on Thursday.
Ukraine's envoy to European security body the OSCE went further at an emergency meeting in Vienna.
"What we registered is a direct invasion of the Russian military into the eastern regions of Ukraine," Ihor Prokopchuk told reporters.
Reuters earlier corrected a report that quoted Poroshenko as saying there had been an "invasion of Russian forces."
Ukrainian officials raised the alarm just hours after a pro-Moscow separatist leader said Russian soldiers were fighting alongside rebels inside Ukraine.
East Ukraine rebel leader Alexander Zakharchenko said Russians had taken arms with separatist forces in the restive east of the country.
"Among us are fighting serving soldiers, who would rather take their vacation not on a beach but with us, among brothers, who are fighting for their freedom," he said in an interview posted on the Internet site of a Russian television station.
Up to 4,000 Russian volunteers are also fighting alongside the rebels, Zakharchenko claimed. A Ukrainian military source told Reuters that Russian-backed separatists had seized the strategic high point of Savur-Mohyla.
Ukrainian troops, meanwhile, had withdrawn from the strategic town of Novoasovsk "to save their lives," the country's defense and security council said on Twitter. Russian troops and separatists forces had combined to launch a counter-offensive in the region, the council added.
A pro-government militia fighter also reported that separatists and Russian soldiers had entered the town of Novoazovsk, which is on the road that connects Russia to the Crimea peninsula. Crimea was annexed by Russia earlier this year.
"There is military equipment in Novoazovsk which came across the border two days ago from Russia," a fighter from the so-called Azov battalion, which supports the Ukrainian army, told Reuters by telephone.
"The equipment is carrying the flags of the DNR (Donetsk People's Republic) rebels but they are regular Russian forces," said the fighter, who did not wish to be identified.
Russia's ambassador to the OSCE, Andrey Kelin, denied there had been an invasion or any sort of incursion.
"No Russian forces are crossing in any point the border of Ukraine" aside from a small group of around 10 Russians soldiers who had "unintentionally" crossed the border a few of days ago.
French President Francois Hollande, meanwhile, said it would be "intolerable and unacceptable" if it was proved true that Russian troops had entered Ukrainian territory.
"Europe will maintain (sanctions), even increase them if the escalation increases. I don't want it because it is neither in Russia's nor our interest ... Russia cannot hope to be a 21st century power yet not respect the rules," he said in Paris.
U.N. Secretary-General Ban Ki-moon echoed concern for the region and said Ukrainian and Russian leaders should continue recent talks that began Tuesday in Minsk.
"All must do their part to contribute to the peaceful resolution of this conflict, in a manner upholding Ukraine's sovereignty and territorial integrity," a Ban spokesman said.
Reuters contributed to this report.