MOSCOW — Highlighting a new terror threat to Russia raised by its air campaign in Syria, security officials said Monday they thwarted a planned attack on Moscow’s public transit system by militants trained by the Islamic State group. Sunday’s arrest of several terror suspects — some of whom, intelligence officials say, were trained by Islamic State militants in Syria — has brought back memories of the string of deadly bombings that struck the Russian capital just a few years ago. President Vladimir Putin has acknowledged the new danger, but insisted that launching the Russian air strikes in support of Syrian President Bashar Assad’s army is necessary to fight militant groups before they pose an even bigger security challenge to Russia. “If we just stood by and let Syria get gobbled up, thousands of people running around there now with Kalashnikovs would end up on our territory, and so we are helping President Assad fight this threat before it reaches our borders,” Putin said in an interview with Rossiya state television broadcast Sunday.