"I look forward to reaffirming the unwavering commitment of the United States to Ukraine and its people when I welcome President Poroshenko to the White House next month," Obama said in a press briefing.
Early this month, the two leaders agreed in a telephone conversation to hold a bilateral meeting as part of the NATO summit, which starts in Wales on September 4.
On Thursday, Poroshenko said Russian troops had entered Ukraine as NATO released images that it claimed showed Russian artillery on the Ukrainian territory.
Obama in his address referred to the NATO shots and accused Russia of "encouraging violence" in Ukraine and violating its sovereignty and territorial integrity.
Russia denied the allegations as it has any previous accusations of the involvement in the Ukrainian crisis. Moscow has repeatedly urged Kiev to stop the bloodshed and establish a direct dialogue with representatives of Luhansk and Donetsk regions.
According to the United Nations, more than 2,000 people have died as a result of the confrontation, and over 4,950 people have sustained injuries in the fighting.