Ukraine says a convoy made up of 32 tanks, 16 howitzer cannons, and 30 trucks of troops and equipment crossed the border into the rebel-controlled Luhansk region, Sky News reports.
"The deployment continues of military equipment and Russian mercenaries to the front lines," Ukraine military spokesman Colonel Andriy Lysenko said in a televised briefing.
Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic, said the Ukrainian army had launched "all-out war" on rebel positions, the Russian news agency RIA said.
Kiev denied this, saying the army remained in agreed positions.
"We refute these allegations ... we're strictly fulfilling the Minsk memorandum (on a cease-fire)," Ukrainian military spokesman Vladyslav Seleznyov told Reuters by telephone.
The fighting is along the fault lines that have been extended by Russian-backed separatists over the last two months.
Russia is also charging Ukraine with deliberately shelling a school, although The Interpreter reports that Moscow has not provided evidence and public information does not point to Ukraine military positions.
Representatives of the separatist regions earlier put out a joint statement calling for a redrafting of the Minsk deal, which established a cease-fire in exchange for Kiev granting "special status" to eastern territories.
Rebels say Ukraine has violated the deal by seeking to revoke a law that would have granted eastern regions autonomy.
Kiev says this was a consequence of Sunday's separatist leadership elections, which it says go against the agreement.
At least 4,000 people have died during the conflict.
Source: Business Insider