Ukraine on Tuesday accused Russian forces of attacking its soldiers fighting pro-Kremlin insurgents in the ex-Soviet republic's separatist east.

"In violation of all prior agreements, Ukrainian military units were attacked in the north (of the war zone) by regular units of the Russian armed forces," Ukrainian military spokesman Andriy Lysenko told an hastily convened press conference in Kiev.

The charge came only hours after Moscow denied claims by Kiev of about 700 new Russian soldiers crossing over into eastern Ukraine to help the rebels' latest advance on government troops.

It also marked the first time that Kiev has accused Russian soldiers of attacking its forces since the warring sides signed an often-violated September 5 truce.

Lysenko said Tuesday's attack happened in the separatist Lugansk province -- the region the Russian forces had allegedly entered the day before.

The claims are likely to add to tensions surrounding high-stakes talks in Berlin on Wednesday aimed at stemming the latest spike in violence in a conflict that has claimed more than 4,800 lives.