Russia and US are the world's two biggest nuclear-armed countries in 2025, holding nearly 5,500 and 5,200 nuclear warheads respectively, according to a report by Stockholm International Peace Research Institute (SIPRI). China ranks third with 600 warheads while France and UK rank next with 290 and 225 warheads. India and Pakistan, with 180 and 170 warheads, rank sixth and seventh.