Ukrainian President Petro Poroshenko called a snap meeting of his security council Thursday, declaring that Russian forces had invaded the country.
Poroshenko dropped plans to attend the inauguration of Turkey's newly elected President Recep Tayyip Erdogan and summoned the council as pro-Russia separatists tightened their grip on the town of Novoazovsk in southern Ukraine, opening a new front in the months-long battle with Ukrainian government troops.
"Today the president's place is in Kiev," he said.
"I have decided to cancel my visit to Turkey because of the sharp escalation of the situation in the Donetsk region ... as Russian forces have entered Ukraine," he added.
Two columns of Russian tanks and military vehicles fired Grad missiles at a border post in southeastern Ukraine, then rolled into the country Thursday as overmatched border guards fled, a top Ukrainian official said.
Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, said the missiles from Russia were fired about 11 a.m. About an hour and a half later, the vehicles entered Ukraine from Veselo-Voznesenka, just across the border from Novoazovsk in the Rostov region in Russia.
A top NATO official said at least 1,000 Russian troops have poured into Ukraine with sophisticated equipment and have been in direct "contact" with Ukrainian soldiers, resulting in casualties, the Associated Press reported. He called that a conservative estimate and said an additional 20,000 Russian troops were right over the border in Russia.
Novoazovsk had until recently escaped the conflict raging further north in Donetsk but came under heavy fire this week as rebels pressed into the city.
Ukrainian officials would be calling for an emergency meeting of the United Nations Security Council and a meeting of the European Commission to discuss the escalating crisis, Poroshenko's media service said.
As tensions rose in Kiev, Russian state television broadcast an interview with a separatist leader who claimed that thousands of Russian citizens were fighting alongside the separatists in southeast Ukraine as volunteers.
Alexander Zakharchenko, prime minister of the self-declared Donetsk Peoples Republic, said many of the Russians who had joined the separatists' ranks were retired military personnel or soldiers on leave.
"They are fighting with us, understanding that it is their duty," he said.
Arseniy Yatseniuk, Ukraine's prime minister, appealed to the West for support in the crisis, urging the United States, the European Union and the G-7 countries to freeze Russian assets until Moscow halted military support for the separatists.
Ukraine accuses Russia of sending troops and military equipment across the two countries' mutual border to support the separatists, who have lost ground to government troops in recent weeks but have opened up a new front in the southern Donetsk region bordering the Azov and Black Seas. Russia has repeatedly denied military involvement in the conflict in southeast Ukraine.
French President Francois Hollande said the presence of Russian troops in eastern Ukraine was unacceptable. "If the escalation continues, European Union sanctions [against Russia] will remain and could even be strengthened," he said.
German Chancellor Angela Merkel called Russian President Vladimir Putin on Wednesday to demand an explanation of reports that Russian troops had invaded Ukraine, according to her office.
The Kremlin media service said Putin had informed Merkel that Russia was planning to follow up on the delivery of humanitarian aid to Donetsk last week with another aid mission to the embattled region.
Zakharchenko said accusations by Kiev and the West that Russia had invaded Ukraine were a ploy to justify the Ukrainian military's onslaught against separatist strongholds in Luhansk and Donetsk.
Between 3,000 and 4,000 Russian citizens had fought as volunteers alongside the separatists in southeast Ukraine, he said.
"Some have gone home, more have stayed," he said.
Among the volunteers are Russian servicemen on leave, who choose to give up their vacations to join "their brothers in the fight for freedom," he said.
Zakharchenko admitted that some of the Russian fighters had been killed in battle, adding weight to a spate of media reports describing the secret burials of soldiers in Russia. The Kremlin has dismissed the reports as "'rumors."