Kiev and its Western backers accused Moscow of sending Russian troops and armor to fight alongside rebels in eastern Ukraine, dimming hopes for a diplomatic solution to the crisis and prompting the U.S. and Europe to threaten new sanctions.

Ukrainian President Petro Poroshenko convened an emergency meeting of his security chiefs Thursday and vowed that "Ukraine can defend itself."

He dispatched troops to meet the Russian-backed advance as his government ordered the reinstatement of a draft this fall—though it pledged not to send conscripts into the war zone.

But while officials in Kiev called on Western capitals for support, including military aid, the initial reaction from the U.S. and Europe included expressions of frustration with the Kremlin but no immediate action.

President Barack Obama ruled out a U.S. military response but said that the escalating incursion into Ukraine would bring more costs for Moscow.