Russia has vowed to hit back against any U.S. strike on Syria, raising the prospect of a military clash between the countries and escalating the already high tensions between Russia and the West. Nikki Haley, U.S. ambassador to the U.N., said Monday that the U.S. was “prepared to act” in Syria if the U.N. failed to implement a monthlong ceasefire in the besieged Eastern Ghouta enclave. On Tuesday, the head of Russia’s general staff, Valery Gerasimov, said Russia would be prepared to retaliate for any U.S. strikes in Syria that threatened Russian servicemen. “In case the lives of our soldiers are under threat, the Armed Forces of the Russian Federation will take retaliatory measures both against missiles and against the delivery vehicles used,” Ria Novosti quoted him as saying.