Newly arrived tanks have been spotted travelling towards the eastern Ukrainian city of Donetsk.
An Associated Press cameraman filmed at least six tanks driving through Shakhtarsk on their way to the rebel-controlled city.
Last week, Ukraine’s military accused Russia of sending more tanks and troops into the east of the country to support pro-Russian separatists, accusations that Russia has denied.
Intensive shelling around Donetsk on Sunday increased the strain on a two-month old ceasefire between Kyiv government forces and pro-Russian separatists.
Reuters reporters inside the city said the shelling sounded more intense that at any time since early October.
The strikes appeared to come from territory held by both government and rebel forces.