President Petro Poroshenko announced in a statement posted on his website that he has dissolved parliament and called for snap elections on October 26.
He said the move was in coherence with the Ukrainian constitution, noting that the ruling coalition collapsed several weeks ago.
"Many deputies who are in the Rada (parliament) are direct sponsors or accomplices, that is to say allies of the militant separatists," Poroshenko said.
The announcement came a day ahead of a summit that includes both Poroshenko and Russian President Vladimir Putin, and could be aimed at pressuring Ukraine into seeking a negotiated end to the conflict rather than a military victory.
Over the past month, Ukrainian forces have made substantial inroads against pro-Russia separatists in eastern Ukraine, taking control of several sizeable towns and cities that had been under rebel control since April, when the clashes began.
But the advances have come at a high cost — more than 2,000 civilians reportedly killed and at least 726 Ukrainian servicemen. There is no independent figure for the number of rebel dead, although Ukrainian authorities said Monday that 250 rebels were in fighting around Olenivka, a town 25 kilometers (15 miles) south of Donetsk.
Earlier Monday, a Ukrainian official said a column of Russian tanks and armored vehicles entered southeastern Ukraine — a move that brings the conflict to an area that has so far escaped the intense fighting of recent weeks.
Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, told reporters that the column of 10 tanks, two armored vehicles and two trucks crossed the border near the village of Shcherbak and that shells were fired from Russia toward the nearby city of Novoazovsk. He said they were Russian military vehicles bearing the flags of the separatist Donetsk rebels. The village is in the Donetsk region, but not under the control of the rebels.
The Ukrainian National Guard later said two of the tanks had been destroyed.
In Moscow, Russian Foreign Minister Sergey Lavrov said Monday he had no information about the column.
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