Ukraine’s president has cancelled a visit to Turkey, citing “Russian troop deployments” in the country’s east, the BBC informs.
His remarks came as pro-Russian rebels opened a new front in the south by seizing the coastal town of Novoazovsk.
Nato says it has detected a significant increase of Russian arms being supplied to the rebels over the past two weeks.
Russia has denied that its forces have crossed Ukraine’s border. At least 2,119 people have been killed in four months of fighting.
The United Nations Security Council was to hold an emergency meeting in New York on Thursday to discuss the crisis.
Nato Brigadier General Niko Tak told the BBC that there had been a “significant escalation in the level and sophistication of Russia’s military interference in Ukraine” over the past two weeks.
“[Nato has] detected large quantities of advanced weapons, including air defence systems, artillery, tanks, and armoured personnel carriers being transferred to separatist forces in eastern Ukraine,” he said.
“Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favouring the Ukrainian military.”
More than 1,000 Russian troops are operating inside Ukraine, both supporting the separatists and fighting on their side, according to Nato