The tanks rolled through a border checkpoint at Luhansk controlled by pro-Russian separatists, to join fighting in eastern Ukraine, said Ukrainian Interior Minister Arsen Avakov.
Poroshenko also explained a peace plan for southeast Ukraine to Putin, according to Kremlin spokesman Dmitry Peskov.
"The president of Ukraine informed Vladimir Putin about his plan to settle the situation in the south-east of Ukraine. The presidents also discussed the development of bilateral relations," Peskov added.
"We are continuing our work towards a political and diplomatic resolution of the crisis in Ukraine and the conflict between Ukraine and Russia," acting Ukrainian Foreign Minister Andrei Deshchitsa said Thursday. "The contact group has met. It worked out conceptual approaches towards implementation of this plan. We are now waiting for Russia's response to his document."
The details of Poroshenko's proposal were not made public, but the Ukrainian Foreign Ministry announced it awaited Putin's reaction to the plan. In his June 7 inauguration, Poroshenko called for disarming the separatists, amnesties, a dialogue for peace, and early elections in the Luhansk and Donetsk regions torn by separatist activity.