Iran has issued a major nuclear warning, saying it could consider enriching uranium to 90 percent — weapons-grade level — if the United States or Israel launches fresh attacks. The threat comes as the fragile US-Iran ceasefire appears increasingly unstable, with Donald Trump describing the truce as being on “massive life support” after rejecting Tehran’s latest proposal for a permanent settlement. Iran is already enriching uranium to 60 percent purity, but experts say a jump to 90 percent would place Tehran dangerously close to having material suitable for a nuclear weapon.