“Preparations for conventional conflict between Russia and Ukraine are accelerating and the likelihood of open war is increasing rapidly,” The Institute for the Study of War (ISW) noted Thursday. The think tank’s stark warning comes after Russia’s state security service accused Ukraine of sponsoring terrorism plots inside occupied Crimea that killed two Russian soldiers in separate incidents.

Putin escalated the accusation, saying ominously, “We obviously will not let such things slide by,” according to the New York Times. Ukraine’s government has denied any incursion into Crimea and dismissed Putin’s claims, but the country did put its troops on the highest state of readiness.

“Putin may be seeking to trigger a political crisis in Kyiv designed to topple Ukrainian President Petro Poroshenko,” ISW further speculated. “The situation for now, however, is moving clearly in the direction of open conflict between Ukrainian and Russian forces in Donbas or elsewhere in Ukraine.”

Russia illegally annexed Crimea in 2014, taking advantage of civil unrest in Ukraine. Russia also sponsors an active separatist movement in eastern Ukraine, and has deployed its own troops under false uniforms to fight the Ukrainian military. Russia is under international sanction by the U.S. and EU for its behavior in Ukraine.

Putin’s current rhetoric is disturbingly similar to statements he made in August 2014 before invading Ukraine the first time. Putin said he was merely “stationing” troops on the border to protect ethnic Russians, before invading Crimea and illegally annexing it.

Ukraine has a seen a prolonged increase in Russian separatist fighting in the disputed eastern Ukrainian states and suffered historic casualties in July. Ukraine’s government says 57 percent of its casualties were caused by weapons banned under an agreement between Ukraine and Russia.

Independent reports indicate Russia is massing large forces on the Ukrainian-Crimean border. Crimean Tartar activists are reporting large amounts of Russian heavy weaponry moving through Crimea toward Ukraine. Reports also indicate Russia is harboring a Nanuchka-class missile ship in Crimea. The Russian Ministry of Defense portrays the mobilization as a routine troop rotation, but independent reports dispute that interpretation.

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